When you open the game, there are a few settings you will want to change.
Click the Settings button and then go to Graphics. Set your view distance quality to Low. This allows you to see exits and stuff through walls in some levels. You should also set your shadow quality to Low. This removes basically every shadow which makes it easier to see in many levels. Afterwards, scroll down to the bottom and change your gamma. I like to have it all the way up, but if that's too bright you can lower it. It's best to have it above 75%. Now you need to click on the Game tab and turn off the VHS effect. This makes it much easier to see because your camera won't be distorted. Finally, turn off head bobbing. This makes it easier to do specific alignments because your camera will be stable.
There is one basic glitch you need to know before you start which is called bhopping. Bhopping is when you constantly jump while sprinting, but let go of your sprint key while in mid air. If you jump while moving, you will keep that momentum in the air, but you can regain stamina while you are in mid air if you aren't sprinting. You can get between two places much faster than running because you won't run out of stamina (usually).
You can also change any keybinds if you want. The only one that will really matter later is the interact keybind. Some glitches and strategies are easier with an autoclicker, and for that reason, I like to use right click for my interact key. Another benefit of doing this is that you have a use for your right middle finger, which would otherwise be resting on right click for no reason.
There are two routes you can take from here:
1. Do your first playthrough on singleplayer, as mentioned below. This is harder and probably more boring but you will get many more achievements.
2. Do your first playthrough on multiplayer. If you want to do this I recommend doing it with the same people for the entire game. However you will only get one achievement from this.

Start by making a singleplayer save. The goal of the level is to find the ladder to get the key and then find the door. This is where low view distance quality comes in handy - you can see through walls that are too far away from you. Use this glitch to find the ladder, pick up the ladder pieces, then find the door. When you get to the pitfalls, walk into it sideways so you don't have to worry about balancing. Once you lose your balance, you can just walk to the other side without having to worry about balancing. When you get to the exit, you can AFK until you get the Survivalist achievement.

If you have followed the steps of the last section correctly and already have the Survivalist achievement, simply go into the vent with the arrow and smiley face to get Escape Level 0.

The bacteria in level 0 poses literally no threat if you are fast enough. It waits about five minutes (depends on difficulty) to spawn. When it does spawn, you will hear three knocks, and it moves around by teleporting. You will hear knocking every time it teleports.

This item is useless for singleplayer but I'll go over it anyway. If you left click while standing next to the pitfalls, you can drop the rope down. Other players can press F while looking at it to climb the rope. If you lean after dropping a rope you can move around and other players can still climb the rope. This is useful so you can go inside the vent with a rope dropped and not have to do anything. Ropes are only found and used in level 0.

This is the first achievement that will probably take more than 30 minutes. The Habitable Zone has four different parts (but you only have to do three to get the achievement).
First, you can get this achievement faster by going back into level 0 through the vent, but it's not really any faster because you still have to do the whole level. This also applies to many other levels that have exits leading backwards or to The Hub.
The first part is a safe level. Go into the red door on the opposite side of the level, count the amount of cars of each color, and input the colors into the elevator (order doesn't matter)
The second part is what I consider the first section where you can die if you know what you're doing because of the skin stealer that roams around the level. The goal is to get four keys to open a door. There are a total of six rooms that can have keys. Two of them are on the left side going out of the elevator, two of them are on the right side going out, one of them is in the middle of the skin stealer area, and one of them is behind the skin stealer (I call this the far room). If you check every room except for the far room, you can just die or restart the level. Keys save on death and randomly spawn whenever the level is loaded. Once you have all four keys, simply interact with each lock within about 5 seconds of each other to open the door.
The third part has a ton of small rooms and an elevator you need to find. There is also a skin stealer here. To find the elevator, start by going forward and right out of the supply room. The right door in the corner will either open to the elevator or be locked. If the door is locked, go left and down the ramp, and then go forward and left until you get to that corner. If that door is locked, it will be a little harder. Now, you have two options:
1. Use low view distance to see the skin stealer through walls, as it tends to camp the third floor quite often. Wait until you see it already on the second floor or going up to the second floor. After that, go down the ramp, and go forward and right. The door in the corner is the final elevator spawn.
2. Restart the level and check the first two spawns again (it randomly generates every time the level is loaded).
The elevator will lead to a big, open room with different ambience from the rest of the level. Simply go into the door with the exit sign to get the achievement.

Flashlights are one of the most useful items in the game. They are useful in many dark levels and are also very common. They can be found in almost every level, including:
- Level 0 second exit
- Level 1 first three parts
- Level 2 before dark zones
- Level 3
- Level 4
- Level 5 first two parts
- Level Fun
- The End
- Level 94 castle
- Level 6
- Level 8
- Level 0.11
- Level 9
- Level 10
- Level 0.2
- Level 188
- Level 37.2 clump zones
- Level Fun+
- Level 55.1

Almond water is undoubtedly the most useful item in the game (unless you are playing on easy mode). When you drink almond water, it increases your sanity by 50% (sanity never goes above 100%). Almond water can be found in almost every level, including:
- Level 1 (all 4 parts)
- Level 3
- Level 4
- Level 5 first two parts
- Level Fun
- The End
- Level 94
- Level 6
- Level 7
- Level 8
- Level 0.11
- Level 9
- Level 10
- Level 0.2
- Level 6.1
- Level 188
- Level Fun+
- Level 52
- Level 55.1

In level 1 part 2 (key section), the skin stealers will spawn either disguised or already in their gray form. I have no idea how the game decides how they spawn. There will be one skin stealer if you are playing with one or two players, and there will be two skin stealers if you are playing with three or four players. They will walk around but will start running if they see or hear you. You run slightly faster than skin stealers do, and the easiest way to avoid them in this level is just to hide in a room. Skin stealers can't open doors. You can also hide in the elevator because the skin stealer is too tall to fit, but it has a chance of getting stuck and forcing you to restart.
In level 1 part 3 (elevators), there will only ever be one skin stealer, which will spawn as disguised or undisguised randomly. It always spawns on the bottom floor. If you get chased, you can just press F on the door and you will instantly hide in it. However, there is a chance that the skin stealer can go INSIDE the room with you. In that case you just have to open the door and be ready to run to another one. A lot of the time the skin stealer can get stuck if it tries to go into the bottom floor elevator, but it rarely will go inside.

Once you go into that door, you will end up in The Hub. The Hub is a tunnel connecting to lots of backrooms levels, and a place where you can regroup. You will see a lot of doors leading to different levels (list at the bottom), but you need to go to the door on the left side. This will lead to a door with a security terminal. Use WASD to find the code that matches with the code on the rightmost column. If you did it right, the column will turn green, and if you did it wrong, everything will turn red and it will say ERROR. Once all three columns are green, the door will open up, leading to a dark MEG base. Directly to the right of the door is the power, where you simply have to click five times. Wait until the switch is all the way down before clicking again. Once the power is on, you can use the computers. The password is "itheardyou" (it can be found on a note at the far side of the room). Click on the Gate.exe file and open the high security gates. Now, just go back to the tunnel and into the "1" door to get the achievement.
Levels connecting to The Hub:
Level 0 (Manila Room)
Level 1 (Smiler Section)
Level 4
Level 37
Level 94 (Main Part)
Level 7 (Above Water)
Level 9 (Main Part)
Level 3999
Level 188 (Main Floor)
Level 52

The smilers will always follow you (even if the lights are on) but can only kill you when they are off. They will stop following you if you go down the ramp at the start, go up the ramp to the left, or go inside one of the side rooms. The smilers are about the same speed as you.

After going into the 1 door, open the gate on the left. If it doesn't open when you flick the switch and makes a high beep, go back into the hub and revisit the previous section of the guide. This level is all about timing. The lights will turn on and off at random intervals, and a ton of smilers will spawn when the lights are off and kill you if you are in the darkness. The ramp to the left is useless and just leads to a dark garage area with nothing that you can interact with. Start by going to the room to the right, it is closer to the ramp leading into the room. Keep hugging the wall on the right side until you are in the room closest to the wall on the other side. Check the door on the left. If it leads to a hallway with pipes, go in. If it leads to a small empty room, go to the room across from it and then check the right door.
Now you are in level 2 (Pipe Dreams). There will be two paths, station and fun. The fun path is for later in the game, so go to station. It is all the same for about a minute (time it takes if you are bhopping) until there is a flashlight and darkness. After that, bug spray will shoot out of a pipe, and then the darkness will end. Once you see the text on the bottom of your screen you will have infinite stamina and you can stop bhopping. Keep your eyes peeled for a room to your left with brick walls and go into the gate to get the pipe dreams achievement.

The smiler in level 2 has three different spawns. First, if you go to fun before doing the station route then the smiler will spawn in the darkness and go towards you. If you go to station, then the smiler will spawn at the darkness once you get to the end of it and it will chase you to the station exit. If you go to fun after doing the station route, the smiler will spawn at the darkness once you get to the end of it and chase you to the fun exit. The smiler in this level is a lot faster than you, so don't even try to outrun it further than the exit.

This is the first truly hard level, and it took me 2 hours on my first playthrough. The Electrical Station is separated into 3 parts. To start, walk up to the panel with 9 red lights.
The first part, the East Substation, is a complete joke. There are no entities at all, you just have to find three breakers.
The second part, the West Substation, introduces Hounds. Hounds have a faster running speed than you, but they can be blinded using flashlights, which stuns them for a few seconds. They do have some special traits based on the difficulty but those will be explained in the Hounds section.
There are only two hounds in the west substation. The first one is in a hallway, and you just have to go straight into the left door and close it. You can also just blind it until it leaves. The other hound is in the final room on the left side, but it will run out of sight and away from any breaker spawns if it chases you. The two breaker spawns in this room are right in front of the door and then on the left side. You can skip this room if you're lucky enough (10% chance) but it's not even worth restarting the level because the hound is very easy to avoid. Restart the level after getting all 3 breakers.
Finally, the Generator is the annoying part. First, you are going to go into the gate, aggro the hound, and run out of the gate. Hounds can't go through gates, and this will make the hound despawn. Almost every room in this level has a hound in it, and it is very hard to survive if you don't use the strategy I am about to explain. First, open the door and wait for the hound to chase you. Next, close the door while it is chasing you so the hound is stuck trying to chase you through the door. Once the hound leaves the door (when the growls get quieter), run to the next room. The hound will not chase you for the time it takes for it to go back to its spawn location + around 5 seconds delay.
For the path of the level, I would recommend just looking at a video of the level. Explaining the path would be a little overkill, and it's better to memorize it if you're going for all achievements.
To finish off this hell of a level, go back to the beginning, and go to the panel again. All 9 lights should be green now, and the gate next to it will be open. Go in the door and you will finally get your well-deserved achievement.

The hounds are one of the most annoying entities in the game. First of all, they run faster than you (unless you are playing on easy difficulty) but can be blinded with a flashlight. Blinding the hounds is very janky, and sometimes just doesn't work at all. Plus, if you are not on easy or normal mode you can only blind hounds once before their eyes just become immune to flashlights.

Energy bars will give you a temporary stamina boost, allowing you to run for longer. These are mostly useless because of bhopping, but they do restore 25% of your sanity. They are found in many levels, including:
- Level 3
- Level 4
- Level 5 first two parts
- Level Fun
- The End
- Level 6
- Level 7
- Level 8
- Level 0.11
- Level 9
- Level 10
- Level 0.2
- Level 6.1
- Level 188
- Level Fun+
- Level 52
- Level 55.1

Level 4 is the Abandoned Office, and it is a huge break after completing the Electrical Station. Once you go through the first door, there are two paths you can take. Going forward will lead to the hub door. The hub door is pretty useful for this level because you can go back here and collect supplies if you are running low in another level. The main path is going into the hallway to the right, and then going in the door to the right of that. You will end up in a room with objects scattered around and four vending machines. Simply count the objects that the vending machines have on them, and put the number into the machine. Once all the numbers are correct, the door will open. In this next room with all the cubicles, you can go into either of the two doors at the top left, and then a few more doors will take you to a button. There are four cameras that you need to avoid or else the door to the exit will be locked. Go in the door to the right then to the left to go around the first camera, and then go in the door straight ahead to avoid the next. After that, you just need to go into the door to the right to avoid two cameras in the hallway that is in the door forward. Now you just need to go into the last door and go into the hotel themed staircase. If you can't go in the door, you need to use a crowbar to go through the vent or just backtrack to the start to press the reset cameras button. Once you escape the level, you will get the achievement.

Juice gives you a ten second boost to your walking speed, making it faster than sprinting. Similar to energy bars, they restore 25% of your sanity. They are found in many levels, including:
- Level 4
- Level 5
- Level Fun
- Level 94 castle
- Level 6
- Level 7
- Level 8
- Level 0.11
- Level 9
- Level 10
- Level 0.2
- Level 6.1
- Level 188
- Level Fun+ main part
- Level 52
- Level 55.1

The crowbar is basically useless. It can be used to open vents, but there are not many vents that require a crowbar to access. One of them is in level 4 and allows you to go back to the reset cameras button without having to navigate backwards through the level, but it's slower than going backwards because you have to crouch almost the entire way. Another one is in the Manila Room and it just leads back to level 0. Also, there are no crowbars in the Manila Room, so you would either have to bring a crowbar from level 4 without dying or utilize the hub doors. Finally, the only place where the crowbar could be remotely useful is the Abandoned Outpost, where you can travel around with vents, but there are many crowbars inside the level, and the vents are not even that safe because the wretch can kill you even if you are inside a vent. It can not be used as a weapon by players (but can be used as a weapon by wretches in Level 9 and the Abandoned Outpost).

Level 5, the Terror Hotel, is another hard level, but definitely not as hard as level 3. This level, like level 3, is separated into three parts, which are the three areas from backrooms lore. This is the first level where sanity might actually become a concern, as you will run out of sanity in just 5 minutes and 33 seconds.
Part 1 is the Main Area. You start in a fancy room with four pictures. To open the door, you have to press the buttons under the pictures in ascending age order (bottom right, top right, bottom left, top left). Get the bug spray and go into the hallway. You need to look for swarms of Male Deathmoths and spray them with bug spray to get moth jelly. There is a dumbwaiter on one side of the room with a note saying "Feed it and they will reward you". Place the moth jelly into the dumbwaiter (make sure you interact with the dumbwaiter, people will often accidentally eat the jelly) to get the keys. There are three locks on the elevator in the middle that need to be opened. You can listen to some relaxing elevator music before going to the actually hard hotel levels.
Part 2 is the Beverly Rooms. Normally, you would have to run around collecting papers while also avoiding the hound and skin stealer that roam around the level. However, the exit door only has three possible codes that can easily be guessed. The codes are 05938, 17564, and 89472. When you get the right code, you will be exited out of the combination lock. Wait until the lock disappears before opening the door, otherwise you will get stuck.
Part 3 is the Boiler Rooms. This level has female Deathmoths, which are giant moths that will chase you if you make noise or shine a light at them. For this level, I would recommend just searching up a guide. The video I used was https://www.youtube.com/watch?v=TQd794qAicw There are two checkpoints in this level, and you can use a thermometer to figure out where moths/hives are. Once you find the pipe hallway, go inside to get the achievement.

Male Deathmoths: Swarms of moths that you can spray bug spray on for moth jelly.
Skin Stealers: A single skin stealer spawns to the left of the lobby of the second part of the hotel. It will always spawn in disguised form.
Hounds: A hound spawns at the end of the hallway to the right of the lobby of the second part of the hotel. After you aggro it once it will just despawn.
Female Deathmoths: Giant moths in the boiler room that will chase you if they hear you or see light from your flashlight. They will either kill you on the spot or carry you to their hive. Walking into a hive also kills you. Moths can chase you out of some hives.
Hiding: In the second part of the hotel there is a bed and closet inside each hotel room. You can hide under the bed and in the closet from hounds and skin stealers.

Bug spray is essential in order to beat the game as you need moth jelly to escape the first part of level 5. However, male deathmoths only appear in level 5 (and in level 9 but they are too high up to be bug sprayed), so it's more worthwhile to just dupe moth jelly if you are playing on nightmare.

If you use bug spray on male deathmoths, they will drop moth jelly. Moth jelly basically combines the effects of energy bars and juice (stamina and speed boost) and also restores around 1/3 of your sanity. Moth jelly is useful in nightmare mode if you have to quickly run from an entity.
Male deathmoths can be found and bug sprayed in:
Level 5-1
Level 5-2
Level 5-3
Level 9 (opposite house from hub house)

The thermometer shows the temperature of your surroundings in degrees Fahrenheit (for some reason) and is only useful in the Boiler Room. Normally it shows as 72F, but it will go up whenever you are close to a moth or a hive. It will always be a three digit number, and sometimes it doesn't even show a real digit. However, you can tell you are near a moth if you see kind of a "melting" effect. You can also just use a guide or memorize where the moths are.

Once you escape the boiler rooms, you will end up back in level 2. Now, you can take the fun path without being blocked by the smiler. This time, pay attention to the right side of the hallway, and run into the small room with the colorful light. This will lead you to level fun.
Level Fun is a birthday party themed level, with tall yellow entities called Partygoers. Sanity drain here is even faster than the hotel, and you will run out of sanity in 4 minutes and 10 seconds. You will spawn in a room with two tables, and you need to go through the hallway. In this first area, there will be a partygoer facing the hallway, so if you step into it, the partygoers will chase you. You can either aggro the partygoers and then hide under a table or you can pop a balloon after going under the tables and then hide. The partygoer will be facing the other way and you can crouch past. The next set of partygoers just requires you to not be dumb, and go left on that one split path and crouch around the partygoer to go around the corner. Now you will be in a bigger room which is definitely the hardest part of the level. First, go straight until you get to the corner. Stay close to the wall. When you get to the corner, carefully move forward across the wall until you get chased. Then, run into the hallway to the left and go under the first table. Now, you can go back and run that way. While you get chased, you need to turn left, turn right, and go under the table. Finally, you can crouch around the last partygoer, and then run into a party room. You will go into a poolrooms-themed area, and just go into a slide. You have to enter the slides from the bottom, going from the top will not work. Going in the slide will give you the achievement.
Note: DO NOT DRINK LIQUID PAIN.

Partygoers will stand still dancing unless they see or hear you or you pop a balloon. Partygoers will immediately go towards the source of a player noise or balloon they hear. They can only chase one person at a time but can kill other people while chasing them. The partygoers will go back to their location after you hide under a table (one of them will actually teleport back). Partygoers can not chase you through the doors to the party room at the end of the level.
Partygoers are the glitchiest entities in the game. Here's a quick list of all the glitches you can do with partygoers.
- Have the partygoers chase you, then go under a table from the side. The partygoers will freeze in place but can still kill people.
- Go behind the table in the first room and stand where the partygoers will see you. This will make all of those partygoers freeze.
- Pop a balloon from a place where the partygoers can't pathfind to you. They will freeze in place and won't chase anyone until you go under a table.
- Crouch behind a partygoer and go right up behind it. You can get the partygoer to go on your shoulder like a parrot, and you can push it around. This glitch is very useful for trolling people.

Liquid pain is one of the most useless items. When you drink it, you die. You can't use it to kill other players or entities. The only time where it would maybe be useful is in nightmare (more info on nightmare section).

Once you get past Level Fun, you end up in another safe level. Just don't go into dark rooms or jump into the abyss. You will start in a rectangular room, which leads to a series of repeating rooms. Go along the right side and look for a room that goes four ways. When you get to the four way room, go right, and then go to the opposite corner in the room with the archways. After this, you just need to walk a little further to reach the exit. Go in the red door to get the achievement.

Level ! (Run For Your Life) is a long hallway with red lights and tons of exit signs where you get chased by entities. This is a trial and error level, and you just need to get muscle memory of the level. You can also watch a tutorial and memorize the path that way. It shouldn't take more than 30 minutes to complete if you are good enough. Once you go in the door at the end, you will get the achievement.

There are actually two hordes of entities in level !. The first one is activated if everyone goes past the doors at the beginning of the level (the main horde). It has a lot of partygoers and a smiler in the back. It will chase you to the end of the level, but there is an interesting glitch where if the horde gets to the end before everyone else does that the level automatically restarts. This usually happens if the loading screen takes a while in multiplayer. The secondary horde activates if one person goes past the second door to the right before everyone else goes past the first doors, and it just releases a dozen partygoers to go kill everyone. This is meant to prevent cheesing the level.

The End is a library level where you have to collect tapes while also staying quiet. The entity, called the "Scratcher" can not see you, but it can hear you. Grab a flashlight since the lights turn off after exactly 90 seconds. If you get chased, you can hide under the tables. There are also fire alarms that you can pull to distract the scratcher if you need to divert it away from a specific area. There are 24 tapes you need to collect (you can see a counter if you turn on the computer) and once you get all of them, the computer will make a distinct noise. When you turn on the computer, the scratcher will go straight towards the top left corner and fly into the ceiling. Now, just go into the ring light behind the computer.

The scratcher is the entity that guards The End. It is blind but has a very good sense of hearing. If you walk or sprint near it then you will be chased, but you can hide under the tables. You can also pull fire alarms to distract them. They go away after you get every tape and interact with the computer.

Going in the ring light in The End will take you to level 9223372036854775807 (the 64 bit integer limit) which is an infinite staircase. There are numbers on each floor that say which floor you are on. The floors count normally, except for floors 100 and above saying infinity, and a random 94 somewhere between 1-100. Climb up until you see an out-of-place 94 (be aware that you might have to climb past the real 94 if you get unlucky, it can spawn as high as 100). Once you get to the fake 94, the ground at the bottom will disappear. Before jumping, I would suggest setting a three minute timer, and starting it when you get the achievement. After jumping, you will fall for a while until you go below floor 0 and get the achievement.

The three minute timer is useful here because level 94 has a day/night cycle. During the night, entities called "Animations" that look like small children will kill you if you are not hiding in a house. Run towards the castle in the distance and go into a house when the timer is at 20-30 seconds. Sanity loss is faster than ever before, taking only 3 minutes and 47 seconds to run out of sanity. After going into a house, you don't actually need to turn the lights off or even close the door. Just go under the bed or in one of the closets. You will not lose sanity while hiding either. Climb the rope under the castle to go to part 2 of level 94.
The second part of level 94 is inside the castle. First, you need to do a little puzzle. There are four circles with shapes inside of them, and four boxes with shapes on top of them. Push the boxes to their corresponding shape and the clown mouth door will open. You will now be in a much bigger room. There is a boss fight that starts when you look away from the clown, but it is very easy. All you need to do is go near the rollercoaster and go near the back (I can't really describe the alignment, image below)
(image from koshkamatew's nightmare video)
Once you are in position, look away from the clown until you hear a noise, and then look back towards where the clown was. The way this boss fight works is that the clown (Animated King) can only kill you from behind, but the only way for the clown to get to you is through the path that you are looking at. Now, just wait 100 seconds, and then go in the rollercoaster. After a short cutscene you will be sent to the next level.

Animations:
These entities represent small puppet children and they chase you around at nighttime. They are faster than you so it is safer to hide from them. They can kill you when you're inside a truck if you go too close to the edge. They will also enter houses if they are able to get in and the lights are on. The best way to hide from animations is to enter a closet or hide under the bed.
Animated King:
This entity spawns in the castle when you look away from the clown. It appears in a random location and chases you. If you look at it then it disappears. This entity will always kill you from behind, which can make for quite the jumpscare.

Level 6 is called "Lights Out" and is one of the darkest levels in all of the Backrooms. You need a flashlight to be able to see anything in this level. I would simply recommend searching up a guide on the route for this level as it can be really confusing. Anyway, this level is separated into two sections. The first section is just running around in the darkness with a flashlight. This section is safe but the wretch walks by at the beginning of the level. This can not kill you in any way, and is just meant to scare the player. Eventually, you will end up at a room with a flickering light and four LiDAR (Light Detection and Ranging) scanners. For the rest of the level, flashlights can not penetrate the darkness. The LiDAR scanner does two things. First, it shows the location of players (green dots) and the entity (red dot) in a decently far range. Second, and more importantly, you can click while holding a LiDAR scanner to fire glowing dots on surfaces. This is how you are going to navigate the level (don't worry, the path can easily be memorized). There is a certain point where the wretch will not go to unless it is chasing a player. This part also secretly slips in a split path. If you go the wrong way, you will end up wasting a lot of time, as this path is just a massive dead end. You can hopefully prevent doing this by hugging the right wall once you go through the specific doorway. After more wandering around, you will find a door to a house. Going inside gives you the achievement.

Another item that is basically essential to beat the game (unless you want to navigate through the second part of level 6 in pitch darkness), but is only found in and useful for the second part of level 6. It will show green and red dots within a certain radius. Green dots are other players, while the red dot is the wretch. You can left click while holding it to fire dots onto the wall, but it only works on the second part of level 6. You can also bypass the dot cooldown by selecting another item and then selecting the scanner again, but the dots will be a random color whenever you reselect the scanner.

Near the start of level 6, you can see a wretch walk by and you will hear a spooky noise. This wretch can't kill you and is only meant to scare the player. In the second part with the LiDAR scanner there is a wretch that patrols the area. This wretch can't see you due to the darkness and as long as it doesn't hear you it won't kill you. It can walk into you without killing you, unlike the Scratcher. The wretch appears as a red dot on the LiDAR scanner. The wretch usually doesn't go past the halfway point (which is the split path area) and also can't go past the room with the LiDAR scanners.

By now, you have gotten half of the achievements, but you are nowhere near half of the game's difficulty. Actually, 29/30 achievements isn't even near half. This part is where you get into the update 3 levels, and you can see a noticeable increase in boring levels and also difficult levels. Level 7 falls into the "boring levels" category, as you are literally just sailing across an ocean with no threat other than falling asleep from boredom. There is not even an entity in this level and the only way you can possibly die is from running out of sanity. You only have to boat to the lighthouse (easier to see with low graphics because it removes the fog), put on a diving helmet, and go into the airlock. You can get to the exit faster by walking into the water and spamming sprint so you can swim really fast.
Part 2 of this level is laughably easy. There is still no entity, and all you have to do is swim 45 degrees left from the airlock. Go inside the cave and swim until you find the exit.

This is the lamest entity in the game. There is a note at the beginning of level 7 telling you to "beware of the thing" but it is impossible to die to the thing. The only time you will ever see The Thing is when the diving airlock is going underwater. It will swim by you but it will never kill you. Therefore, the only way to die in level 7 is by running out of sanity.

The flare gun (or what I like to call it, the "Jumpscflare Gun") is only found in a single house in level 7. Because of this, many people never even see or know about the flare gun. When you left click, it fires a flare into the air. This is mainly only used for navigation in level 7, but you can also just set your graphics to low so you don't even see the fog. It also makes a loud noise, which is why I called it the Jumpscflare Gun.

This is the second update 3 level, and this time it goes into the "hard levels" category. This level has three different sections, but luckily it is quite easy to find the path for all three of them. For the first two sections, the only entity is female Deathmoths, which means you just need to crouch. For both of them you are mostly going to be hugging the left wall. There is a skip here (watch chocler's speedrun guide) to skip the skin-stealer part if you want. The only difference is that you want to go to the ladder to the left near the end of it because otherwise you will skip an achievement. You can fall through the ceiling next to the ladder. If you want to do it legit, just walk past the rocks where you do the skip. Eventually you will find a bridge. The bridge will collapse if you sprint on it or more than one player is on it at a time, so just walk across. Now you actually get to the hard part of the level. The path for this section is mostly hugging the right wall, but there are two skin stealers. They are buffed from the regular stealers (in levels 1 and 5). The differences are:
- Level 8 stealers hold flashlights.
- Level 8 stealers can transform back into hazmat suit form without killing a player first.
- Level 8 stealers are slightly faster than players.
These changes make the level hell, especially in nightmare. For singleplayer, you can just run along the path and restart level if you die, since there is a checkpoint right after the bridge. Once you get to the airlock at the end, you will fall down a small hole and climb up a ladder. There is also a massive pit, and on the other side of it is an area you will go to later. For now, climb the ladder to get the achievement.

Female Deathmoths: These moths in level 8 work the same way as they do in the boiler room except they don't react to flashlights. The moths can fit into the airlock at the start of level 8 part 1, but not into the one at the end.
Skin-Stealers: There are two skin-stealers in the last part of level 8. However, these skin stealers are buffed from the ones in levels 1 and 5. Unless they are chasing a player and are close to them, they will always be in disguised form. If a skin stealer de-aggros on a player, it will return to disguised form with its flashlight. They are also slightly faster than players.

The walkie talkie is used to communicate with other players from large distances using proximity chat. However, this is useless for two reasons.
- You can just send messages in chat.
- You can just go on a Discord voice chat (or just any voice chat).
Walkie talkies can be found in most levels starting from level 8.

After escaping level 8, you will end up back in level 0 (but not really). This area is called the Manila Room, and has three special areas. The first area is to the left of where you spawn and is just the 0 hub door. The second area is to the right and has white walls and red carpet. There is a light you can turn on to make shapes appear on the numbers (1 = square, 2 = circle, 3 = arrow). Fun fact: There is actually a glitch (not sure if it still existed but it happened on my first playthrough of the new levels) where you can not see the shapes on the numbers if your graphics settings are too low. I was stuck on this level for like 15 minutes. The third area is to the left of the second area and has gray walls and floor. There is a locker (the code is square, circle, arrow) with a chainsaw and some other supplies in it. There is also a vent. This vent leads back to the real level 0, so it is kind of useless if you just want achievements. The chainsaw in the locker can be used to tear down the wooden barrier to the dark level 0. When you go in, you will noclip through the floor. This will take you to level 0.11, one of the most annoying levels in the entire game (yes, the achievement says 188, but 188 is only a single room in the level, and 188 is an actual level in the game) The first section is safe and you just need to find arrows leading to a wooden door, but the second section is HELL (unless you are on multiplayer but you have to do it in singleplayer for achievements). There are a lot of doors with barriers that you need to destroy with your chainsaw, and 4 levers around the map. There is also a buffed Bacteria (the entity from level 0). Instead of spawning after 3 minutes and just teleporting around the map and knocking, it limps around SILENTLY and spawns as soon as you walk through the barrier or as soon as you spawn in (there is a checkpoint). The ONLY thing that is done to make the level easier is the vents you can hide in. Below is a description of each of the rooms/lever spawns.
TV Room: The room to the left of the main entrance. Usually has one almond water and is a possible lever spawn.
Shelf Room: The room to the right of the TV room. Doesn't usually have supplies and is a possible lever spawn.
Metal Door: There is a hallway to the left going out of the TV room (which is a hotspot for getting jumpscared by the entity). This hallway has a room with a vent leading to another room. This room has a metal door with a window. This room will always have a lever.
Wire Door: Hallway in front of TV room has a door, and the room has a metal door with a square pattern. The entity can see you through this door but can't pathfind to you. This is a possible lever spawn.
Kitchen: The most risky room to go to. You have to go to the right of the wire door to find this room. It has a counter and dining table with various consumables, and also a fridge with an array of juice and liquid pain bottles. This is a possible lever spawn.
Laundry Room: This room is very close to the gate. Washing machines with a few consumables, and a possible lever spawn.
Gate: This lever is out in the open (luckily the bacteria only goes here if chasing a player). There is always a lever here, and if all four lights are green the gate will open unless you got the dumb glitch.
Now here's the hard part. You have to flick all four levers, with the entity wandering around, and the levers have a TIME LIMIT. After a few minutes the lever will deactivate, so you have to speedrun the level and get to the gate fast enough. This is the level that made me put off beating singleplayer for so long. Once you somehow get through the gate, there is a lever to the right. This lever is only useful in multiplayer (it reopens the gate) so just ignore it and go to the next room. "Level 188" is just a simple puzzle. Input the elevator code (the colored windows in the courtyard, 415314) and then press the button to go to the next level and finally get the escape level 188 achievement.

The chainsaw is used to destroy the barriers in front of doors in 0.11 and also to destroy the massive wooden barrier in the Manila Room. You also have to use a chainsaw on a door in level !-!. Unfortunately it can not be used as a weapon by any player (but can be used as a weapon by Level 10's facelings).

The bacteria in level 0.11 works much differently from the one in level 0. It spawns when you walk through either of the wooden barriers in the level. It will spawn in the long hallway and limp around the area. The bacteria can't hear anything though so it's safe to walk behind it. You can hide from the bacteria by either going in a room or a vent. It is also possible to glitch the bacteria by hiding in these rooms.

Level 9 is called the Darkened Suburbs and is a neighborhood themed level. If your computer isn't very good you could spend up to 30 seconds in the loading screen, which, coupled with the amount of times you will probably die, can get very annoying.
In part 1 of this level, you have to find computers in random houses. Start by going to the center of the map. There will be a small terminal with some houses blinking red. Below is a map of the level (not made by me, see bottom left).
Now I will explain the strategies to get to each house's computer.
House 1: Safe house, simply go upstairs and it is in the last room to the left
House 2: I have never actually seen this house spawn a computer.
House 3: Go to the back of the house (facing the MEG house) and interact with the computer through the wall. It is very hard to get this computer otherwise.
House 4: If there is a computer in this house, GOOD LUCK. First of all, it is very hard to get the wretch outside as it tends to go to the side of the doorway whenever you lure it. So, you have to lure the wretch to the door, watch it go upstairs, and if it goes right, make a run for the first door to the left and LOCK THE DOOR BEHIND YOU. After getting the computer, you can either lean through the right wall in the closet to find the wretch and then leave. You can also just do Esc -> Restart Level. Computers save on death. Also, the hub door in this level is kind of pointless unless you are on nightmare and have no way of duping (or you don't want to dupe)
House 5: Safe house, no strategies required, but it is possible to lure a wretch from another house into this house.
House 6: Try to lure the wretch to the front door, and try to close and open the door until the wretch gets pushed outside. Then you can lure the wretch really far away or you can make it fall through the ground in the street in front of house 8.
House 7: Crouch jump on the small trash can (crouch in mid air while jumping, it lets you jump onto higher objects), then jump onto the fence, jump onto the roof of the house, and then get the computer through the wall/window.
House 8: Lure the wretch out through the garage door and then do the same strategy as house 6.
House 9: Safe house, no strategy required.
Once you have gotten all the computers, go back to the terminal. The gate will be open, and you can go into the MEG house. Simply follow the arrow and go into the basement to get the achievement.

Male Deathmoths: There is a swarm of male deathmoths under the street light in the middle of the level. You can't actually use bug spray on these deathmoths because they are too high up for you to reach.
Wretches: Many of the houses in level 9 have wretches. Unlike the wretch in level 6, these ones can actually see because it's not dark. They also carry crowbars, so they have a different jumpscare. Instead of screaming in your face they just whack you with their crowbar. Wretches can open doors, so you must lock them in order to be safe. These wretches are... kind of glitchy however. In the house to the left of the abandoned outpost, the wretch can exit the house through the garage door and roam around the entire level. You can also make wretches in other houses leave the house they are in by luring them to the doorway and closing the door and constantly opening it to push them outside. It is even possible to lure wretches into other houses. There are also some areas in the ground that are missing collision. They aren't big enough for players to fall into, but you can lure a wretch there to make it fall through the ground.
G. Adams: The wretch in the Abandoned Outpost was previously a M.E.G admin who had run out of sanity while in this outpost. He went through the Wretched Cycle and became a wretch. However, if you fill the shower unit with four bottles of almond concentrate, you can decontaminate the wretch and turn it back into a human. Then, you can take the admin card and use it to access level 10.

The second part of level 9 is called the Abandoned Outpost. The gate at the exit of level 9 can only be opened with an administrator card, so you need to take the administrator card from the wretch inside the outpost. First, you have to watch this presentation, which is actually quite interesting. Once the presentation ends and the door opens, find 3 bottles of almond concentrate. If you're about to die, you can save the concentrate by restarting level before the death animation finishes. Once all four are in the shower unit, lure the wretch onto the footprints in the middle of the room. If you lure the wretch to the footprints before putting in the concentrate, the gates will not close and the wretch will just walk over it. After you trap the wretch, you need to push the shower next to the gate and press F on it. I would recommend looking away while the wretch is taking the shower if your computer isn't very good because the smoke can cause a lot of lag. The gates will open once the wretch is cured to reveal another person in a hazmat suit. Pickpocket them, steal their admin card, and go back the way you entered the level. You will now be back in the main part of level 9. If your computer isn't very good you might end up jittering, but it doesn't really matter. Go back outside of the fences surrounding the house and go to the exit sign marked on the map above. Press F on the red terminal and you should hear two high pitched beeps and then the gate will open. Go inside to go to level 10.
Level 10 is the last main level of update 3, and falls into the Boring Levels category. The first part is literally just running along a road. The only threat is the Facelings (humanoid entities with no facial features) that will chase you and try to kidnap you, but they are a lot slower than you if you bhop the whole way to the barn. You have two choices when you get to the barn.
You can either go the legit path (red) or the fence path (blue). To do the legit path, just run into the wheat field and follow the red windmills. Beware of the faceling that will saw you in half if it gets the chance. The fence path requires you to crouch jump on the fences next to the green plants and run along the fences. I prefer the fence path since it is a lot less confusing but is kind of risky. You just need to get to the parking lot and go into the revolving doors of the arcade to get the achievement.

There are two types of facelings.
Road: The facelings here will spawn on the road behind you and attempt to run up to you to kidnap you. When you look at them, they will run into the wheat. If they catch you, they will drag you into the wheat field and kill you.
Field: A faceling will spawn here with a chainsaw. This faceling chases you just like all the other entities do, but if it catches you, it will kill you with the chainsaw.

This is the first of the update 4 levels. Most of the update 4 levels aren't actually that hard and are just an endurance test. This one is pretty easy however. Walk through the doors and go down a long arcade hallway. Then, go into the elevator and press the buttons in order: 9-3-1. This is meant to be a code which is found on the tickets in the corner, but it's never randomized so it's always 931. Then there is an optional MEG tape that you can skip. The only important thing in the tape is that in the noclip zones in the next level, red and yellow lights mean danger and green lights mean "safety". When you go up the elevator, you will get the achievement.

Another useless item! The ticket simply has the code to the elevator on its texture. The code is always 931, so tickets are useless.

When you escape level 3999, you'll be sent to level 0.2. This level has five rooms with levers you must activate to move on. There is a map of the yellow rooms at the beginning. If you don't want to do the legit route, there is a skip you can do. There's no point in me explaining this because Chocler demonstrates it in his speedrun guide. If you don't want to do it like this, you can use the following instructions. When you noclip into the red rooms, you must immediately be prepared with a flashlight as smilers will come at you from all directions. They will disappear when you shine your flashlight at them for a few seconds. There is also a skin stealer which has unlocked the ability to jump. The traffic lights next to noclip zones tell you if the skin stealer is near their corresponding location in the red rooms, but it doesn't always work (it caused a nightmare death in 0.2) The rooms are located in the same places on the yellow and red rooms, so you can just find a noclip zone near a room and not have too much trouble. You just have to open the door once and then it will be unlocked in the yellow rooms. The yellow rooms are completely safe (except for sanity loss). When you get all five levers, you can click the lever next to the tunnel to block it with a gate. If you try to go past this tunnel without closing the gate, the bone thief will immediately kill you. Run across to the elevator, which will take you to the next level.

This is another very easy level in update 4. You can get supplies in the Snackrooms, but there isn't really a point since you will probably die on the next level anyways. To escape the snackrooms, you must go to the typewriter and click the third key on the top row, the seventh key on the second row, the fifth key on the third row, and the second key on the fourth row (If you are confused, there is a note saying which keys to click). This will open the door to level !-!.

This level is not called !~!, Fancy is just stupid and misnamed the achievement. You can skip the entire level by going back into the exit and restarting the level while exiting. This takes you to the end where you can just go into the exit. For the legit route, you must go forward and to the left and step on a pressure plate. The more players that are in the server, the more people that have to step on the pressure plate. Then you go to the right, flick a lever, and enter the most EPIC chase scene in the entire game. You have to run from the Clump which will chase you through these hotel hallways. This is a memory level, as if you take any wrong turns you will be greeted by a Smiler. A little bit of the way through there is a blue room which acts as a checkpoint. Take the chainsaw and jump in the hole. Saw down the door and continue running down the hallway. Of course, I can't exactly explain where to go because... it would be a little overkill. Watch a video if you need. This level is just trial and error. There are a few fake blue rooms that serve as traps, so don't be fooled. The real blue room can't be seen from far away and will be obscured by a staircase. That room will have the exit door to 188.

The clump is just an entity made of a lot of limbs that chases you. Nothing much to say here. It also only resides in level !-! and some areas of level 37.2.

After escaping level !-!, you aren't going to be getting any more achievements for a while. This is because level 188 doesn't have an achievement (it's already taken by the fake 188), level 37.2 doesn't have an achievement, and level fun+ doesn't have an achievement. Level 188 requires you to find three TVs and push them to the spawn to open the exit door. First of all, there are a few skips you can do to make the level easier. You can watch speedruns to find these skips. Again, I will still be explaining the legit way. Go in the elevator and interact with the up button. It will either take you to floor 2 or floor 3, but it doesn't really matter. Search EVERY ROOM (and every bathroom too) to look for TVs. They always spawn in the same place even after you die. Pushing a TV has some strange quirks. First of all, you can't be holding an item. You can't turn either. You can't use W/S and A/D at the same time, you have to release one before using the other. Also, you can pull the TV backwards when bringing it back to the elevator, because if you have the perfect alignment it can act as a shield to protect against the Aranea Membri. However, it's pretty hard and takes a lot of practice. You can also sprint towards the TV and interact with it to push it really fast. When you get all three TVs onto their letters, the metal door next to them will open. Walk across the bridge, then jump off. If you go to the end of the bridge then you will be chased by an Aranea Membri before jumping off.

Now that you have escaped level 188, it is time for THE LONGEST LEVEL IN THE GAME. Level 37.2 is mostly simple, it's just very long and annoying. There are three sections in the level where you have to turn valves while also avoiding a Clump. These sections aren't that hard and should only take a few attempts at most (unless you're just really bad). You can also beat the entire level without using the painfully slow boats by crouch-bhopping over the Hydrolitis Plague. You can skip the second clump area by going up the right ladder then pressing F11 twice right before hitting the water. Go under the ceiling quick because you will slowly rise. You can skip the third clump area by jumping over the level (watch a speedrun). You can skip everything if you have a flare gun (again watch chocler's guide)

After level 37.2 is ANOTHER LONG LEVEL! Level Fun+ is basically just level fun but harder. There are four sub-sections of this level, and you have to get a key from each.

Section 1: Fireworks
In this first section, you have to throw fireworks at party hosts to unlock doors. When a party host closes its eyes, the corresponding door unlocks. At specific intervals, partygoers will come out of doors and chase you. Just like normal Level Fun, you can just hide under tables. If you die, party hosts that are already stunned will stay stunned. Be careful with fireworks though, as they can be used to kill players. When you stun the last party host, the room with the key unlocks. When you get the key, the partygoer timer stops and you no longer have to worry about the partygoers. You can restart the level after getting the key.

Section 2: Glowsticks
This is the scariest part of Level Fun+. In this part, you have to get four colored glowsticks (all on the bottom floor) and then place them in front of symbols on the wall (all on the top floor). Glowsticks that are dropped will save on death. There is a partygoer that roams the top floor but can't see you because of the darkness. Take an extra glowstick so you can see. I would recommend just using a guide to find the symbols (i will add a map if it's ever created). When you get them all in the right spot, the key can be accessed.

Section 3: Balloons
This part requires you to pop a lot of balloons in the level to find a key. The balloon that the key is in is random, but luckily this level has a lot of hiding spots, so the three partygoers in the level don't even serve as much of a threat. There also aren't many spawns (chocler guide, you know the drill).

Section 4: Simon Says
This part is a complete joke. You go into a very small Level Fun, get a tape, and then complete a simon says game with balloons. It is very easy.

After getting all the keys, you have to do a short arcade game where you use WASD to put the keys in the right colored holes to escape.

Level 52 is the last level you have to do before you can finally get an achievement. You can skip the level by going back into level fun+ and restarting the level while you're exiting. This will teleport you right to the end. For the legit route, simply follow the Partypooper. Make sure to crouch before the Partypooper does (otherwise the partygoers will chase you), but after about five minutes of following the Partypooper you can escape and finally get a well-deserved achievement.

The window entity is just a person that looks through the windows in most rooms in 188. Don't get too close to the window, or it kills you.

This is a large spider-like entity (directly from the wiki) and it guards the upper floors of 188. It walks around and will run very fast towards you if it sees you. However, a major drawback of this entity is that it can't fit in doors, so you can simply leave doors open and you will be safe.

Fireworks can only be found and used in one part of Level Fun+. I already explained this in Escape Level 52.

Glowsticks can also only be found and used in one part of Level Fun+.

The partypooper is the only friendly entity in the game, and it resides in level 52. It will lead you through the level and help you avoid the partygoers. The partygoers can kill the partypoopers. There's also this funny glitch where if the partypooper dies, its body will just be flung all over the place.

This is the final level of update 4, and luckily isn't too hard. You have to grab a camera and take six pictures by using the elevator. The picture you need to take is on a piece of paper. When you take the correct picture, it will switch to the next picture. In order, you have to take a picture of:
- A message on the wall on the first floor
- A section of level 0 on the first floor
- An abandoned 94 truck on the second floor
- The bacteria on the second floor
- Moai heads on the third floor
- The skin stealer on the third floor
If you die, you do not lose any of your pictures. When you get all six pictures, the hatch above the elevator will open up. You can now ride the elevator up. It leads to an overgrown version of level 0. Go into the unlocked door to get the final achievement.
Note: You can get the achievement by going into the entrance back to level 52, but I would recommend just beating the game normally for a better experience.

The camera only is found and can be used in level 55.1. Once you take the required 6 pictures, the hatch on the top of 55.1 opens.

Now that you are done with the singleplayer achievements, it's time to do the multiplayer ones. You can either redo the entire game three times in each difficulty (not recommended) or what I did, joining random servers until you get all three of the achievements. Just join a lobby (4 player recommended), ask for the difficulty, if it is a difficulty you haven't got the achievement for then just hope you can beat the game before getting kicked, running out of time, or the host leaving. You can also play with friends as they might have multiplayer saves that are already at the end of the game, or download a completed save from the discord server. Once you get all three of these achievements, it is time to move on to the last one.

The final achievement is the nightmare achievement. In order to get this achievement, you have to beat the game in Nightmare difficulty with four players. Hounds are buffed, sanity loss is 50% faster, and most importantly, dying resets ALL progress. Everything up to this point probably took around 10-20 hours. This achievement alone could take hundreds. I hope you are ready.
First, be aware of the new duping method. You are able to dupe items by moving them in your inventory very fast (easier if you have higher ping). However, I would recommend dropping other items before duping as this duping method can cause other items to disappear.
Any level with multiple exits can be cheesed in multiplayer as you can constantly walk into an exit when someone's dying to save the run.

If you want to have a chance at being 4 player nightmare mode, you are going to need good players. The easiest way is looking in Fancy's discord server. You could also just find people in game but it will take a lot longer since most of the people who would join your server would probably just be noobs. Once you are in a server with three skilled players, you can move on to part 2.

This next part will contain tips, tricks, and strategies to complete every level in nightmare. There will be tips for 1, 2, 3, and 4 players included.
I will have some general tips in this section:
If anyone has to go AFK, wait until you're in one of the following areas:
Level 0 (pitfalls or manila room)
The Hub
Level 4
Level 37
Level 922 (away from the hole)
Level 94-2 (anywhere the clown can't reach you)
Level 7 (if everyone is wearing diving helmets)
Level 0.11 (before anyone enters wooden barriers, those trigger sanity loss)
Abandoned Outpost (in spawn, a locker, or anywhere once wretch is trapped)
Level 3999
Level 6.1
Level !-! (spawn or safe rooms)
Level 37+
Level Fun+ (main section)
If you're waiting for your teammates to do something, and you aren't the host, then dupe. If your teammates are going to go to where you duped then they can get supplies.
You also want to make sure your inventory is well suited for your current level - I recommend around 6 moth jelly and almond water, then you can drop almond water if you need other items.
Autoclickers are useful in many places, like fun+ balloon section and The End.
If you run out of sanity and are hallucinating to death, you can go into an exit and you will be set to full sanity.

Singleplayer: Memorise the path to the ladder and then from the ladder to the door. You can use any strategy for the pitfalls that you are comfortable with, but if it's kind of risky, then I would recommend memorising the path to the 2nd exit in case you fall.
2-4 players: Have one person get the key. If you have 3 or 4 players, two people can go to the ladder. Once someone crosses the pitfalls, have them drop a rope. You can lean after dropping a rope and it will let you move around with the rope still dropped, so you can go into the vent with the rope dropped and just wait for everyone else to exit.

Part 1:
Singleplayer: I usually would get supplies here in solo nightmare, but it doesn't really matter that much because you can get more items in level 4. You can either go to the red door to get the code, or you can guess it. The codes to guess will be after the multiplayer strategies.
2-4 players: With more players, there is less of a point in getting supplies here, so your best option is probably just to guess the codes. It is faster if a non-host guesses the codes because they only have to use one row and they don't have to worry about the cooldown between guessing each code.
Since red and blue cars are more common than green and yellow in the red door room, some codes are more common than others. Here are the codes below (listed with red and blue before green and yellow):
RBGY, RRBG, RRBY, RBBG, RBBY, RRBB are the most common codes (9/70 = ~12.86% chance each). You have a 77% chance of getting one of these as the correct code.
RRRB, RBBB, RRGY, BBGY are 3x less common than the codes above (3/70 = ~4.29% chance each). You have a 17% chance of getting one of these as the correct code.
RRRG, RRRY, BBBG, BBBY are the least common codes and are 3x less common than the codes above (1/70 = ~1.43% chance each). You have a 6% chance of getting one of these as the correct code.
Part 2:
Singleplayer: It's probably best to go left out of the elevator. You can either open the right locker through the wall or go into the first room and open the locker on the left. The first room on the left also has a lot of items. You can open the two lockers in the next room through the wall, or you can go open them from inside the room. Those are the only 3 key spawns in the left rooms. When you're going around the corner, be careful as the stealer could be right there. In the middle room, check at the table, in the locker on the side, on the table next to that, and the 3rd locker in the set of 4. If you don't have all the keys, keep going to the right. In the next room, every locker can spawn a key as well as some of the tables. The first straight room has a key spawn on some tables, in the isolated locker, and in the 2nd and 4th lockers in the group of 4. Finally, if you still don't have the key, you have to go to the room in the back. Wait for the stealer to go far away or lure it away, then go to the room. Every locker can spawn a key. When going back, lean through the wall to check there and then open the door and lean outside it if the stealer isn't there. If it's safe, run to the middle room and exit.
2-4 players: One person goes to each side. Whoever goes left does the same strategy for those two rooms as in singleplayer, and whoever goes right does the same strategy except with the order of the rooms switched. The first to get to middle room checks all of its spawns. With 3 or 4 players it will be harder to get to the room in the far back because there are two skin stealers.
Part 3:
There isn't really much of a difference in this level with the amount of players, except everyone can follow one person who does this strategy. First, get supplies if you need. If you still don't have a flashlight by now, pick one up here. Quickly go to the right and check the first elevator spawn. If you have low view distance, you can see if it's there through the wall. If the elevator didn't spawn there, then all you will see are the two pieces of the gate and nothing else. You can check the third floor while going down the ramp to the second. If you see just the gate pieces, run to the left to the second floor spawn and exit. If you see the whole elevator, wait near the left. You can either go down immediately or you can wait until you see the stealer on 2nd floor before running down to exit.
The Hub:
Run over to the door after you go down the elevator to go into the hub. There isn't really anything to note about this level, except for the fact that the security terminal can freeze your game as host sometimes and that people can dupe while one person does the hub in multiplayer. Just go back into the 1 door once you run gate.exe.
Part 4:
If there are multiple players, only one person has to do the level because everyone else can wait in the hub exit. If everyone waits in the hub exit and one person goes into the level 2 exit, it tricks the game into thinking everyone is in the level 2 exit. This skip can be used in a few other levels as well. With bhopping this level is easy. You can run to the first room on the right side on the first cycle, then skip the next room to go straight to the exit. If it doesn't spawn on the left door there, skip the next room to go to the other exit spawn.

Just have everyone run towards the left side. If you don't trust someone to not die, you can have another person wait before the darkness so they can get to the exit before the smiler chases.
After the hotel, you can have one or more people run to the left side because it's shorter and you can skip the smiler chase while at least one person runs to the right side.

To open the door, you just have to walk up to the glass panel. Do not interact with it because it could freeze your game.
East substation (first 3 breakers) is not hard at all. There are many places you can get breakers through walls in the east substation, and there is no way of losing the run here other than sanity or a game crash.
There isn't really a point in having more than one person go in west substation. I will not explain the entire layout of the level as it is best to have the level memorised before attempting nightmare, but I will still explain some things to watch out for. West substation has two hounds: one that chases you in the long hallway, and one in the last room. If you have to get any breakers in the last room (you almost always will, it's a 90% chance), look for the hound, and if it's close let it chase you so it goes to the back of the room. The breaker that's farther from the door can be interacted with through the wall in the previous hallway.
When you go in the generator, make sure that you trigger the first hound then run out of the generator until it despawns. It might be easier if multiple people go through the generator. There is a breaker that can spawn in a room with the hound, so you need to have it chase you then get the breaker once it leaves. With 2 people, you can have the hound chase someone else to give you time to get the breaker. The hallway at the end is also annoying because the hound that runs into the vent doesn't despawn in hard or nightmare, so if you're not confident with running into the rooms you can have a second player go to the other side of the gate and have it get stuck on them. Anyone who isn't doing the level should either be duping almond water or waiting in a breaker to not lose sanity.
Here's a low quality map of the entire level:

Get the hub door if you aren't duping and simply go to the exit. You can also drop your flashlight here for an extra inventory slot. You can have someone wait at the button so everyone can run past the cameras but this is slower because one person still has to go through the entire level after everyone else is at the exit.

Complete the painting puzzle as normal then have everyone split up and look for moths with bug spray. Get a moth jelly and have someone dupe it to get the keys. If you're not duping, get as much jelly as you need here and if you want to get more from boiler room, keep the bug spray and you should be able to get 3-4 jelly along the path.
Level 5-2 (Beverly Rooms): Have one person guess the code while everyone else listens and looks for entities. If you're doing the level legit, then have one person go to the right side. Lean around corners before going around them. Hug the left wall, and when you find the skin stealer, alert everyone else (sorry solo nightmare players). By now, the hound is most likely gone, but if it's not, it's best to quickly have it chase someone so you don't get trapped. The doors in the connecting hallway should also be opened. Now you want to run around the entire level so nobody else has to worry about entities and can do the level normally. Make sure you're the last one to go in the exit.
Level 5-3 (Boiler Rooms): Everyone can wait in the stairs while one person does the level. The one person doing the level should have the path memorised.

Before you do anything here, please take note of the risky "Level Fun Skip". One person can use momentum and jump to go into multiple slides so that less people have to get into the exit (it triggers when the number of slides activated is greater than or equal to the number of alive players). However, there is a chance you get glitched and do not actually go into either slide, forcing the other players to get to the slide before you die of sanity so that once you die the exit still triggers and you just lose your items. Alternatively, they can just die if the exits triggered is one less than the players in the server. Anyway, here is the strategy for the actual level. Either decide who should aggro the partygoer or have one person go behind the table and aggro them so they get stuck and don't chase anyone. For the second part one person can crouch jump on top of the chairs behind the table and pop the balloon to freeze those partygoers. Alternatively, you can just crouch around the partygoer. WAIT for everyone to get to the corner before the "run part" because after the partygoers are aggroed they will face the area you have to go through to get there. Choose one person to aggro the partygoers and go under the table to the left and decide which order everyone should go in to run past those partygoers. The first one to run has a high chance of the partygoers aggroing earlier. After that it's just a crouch to the exit. Dupe anywhere in the level if necessary, and check sanity often because it only takes 2 minutes and 47 seconds to die of sanity.

This is another easy level. In 4 player nightmare, the host will always spawn in the same room (turn right to 4-way room), and depending on when you load in (I think it's ping based), the 2nd person will spawn in a hallway with curved walls (opposite side from 4-way room), 3rd person will spawn in one of the sections connecting to the 4-way room, and the 4th person will spawn near the hub exit. It's probably best for everyone except the 3rd person to go to the hub exit. Dupe if necessary before going to level !.

You should figure out which order everyone should go in. You can go to right before the door that's behind a dark section without triggering the partygoers. If you're in the front, you have to react to traps but aren't as close to the horde, but if you're in the back you could possibly get stuck on overturned objects and you will be closer to the horde. Crouch jumping can make many sections easier, but I wouldn't recommend skipping the side hallway in nightmare. The host can also set their FPS cap to 30 so the bookshelves don't fall right before the side hallway.

This part will be similar to achievements 1-26 but will be changed to be the best for 4 player nightmare (with some other strats that don't apply to 4p)
This section is split into multiple parts because steam guide sections have a character limit.
Level 0: Decide who will get the key before running straight to the door. For the pitfalls, just use whatever strategy you would like, as they don't really make that much of a difference for speed.
Level 1-1 (Code): For this level, you can either get all the supplies in this level or just not get any. It doesn't matter that much since you won't be losing much sanity before getting to level 4, which has tons of items.
If you want to guess the codes, do them in this order:
Most common: RGBY, RRGB, RRBY, RGBB, RBBY, RRBB (9/70 = ~12.86% chance each)
Uncommon: RRRB, RBBB, RRGY, GBBY (3/70 = ~4.29% chance each)
Least common: RRRG, RRRY, GBBB, BBBY (1/70 = ~1.43% chance each)
Level 1-2 (Keys): Have at least one person go left and at least one person go straight. Only go to the far room if you know you can get there without dying or you know there is a key there. You can dupe if you get to the exit before everyone else. It may be helpful to say when you collect a key and to keep track of how many keys you have.
Level 1-3 (Elevators): If you do not have a flashlight yet, get one here. Quickly check the first two floors' elevator spawns. If it is not on either of those spawns, have someone set their view distance to low and see where the skin stealer is. Wait until they see it on the second floor and then have everyone run down to the elevator.
The Hub: This level takes about 1-2 minutes. 1-3 people can dupe almond water while another person does the level.
Level 1-4 (Smilers): In this level you can use the "Smiler Skip". One person can do the level while everyone else waits in the hub exit. The glitch works because the game triggers an exit when everyone is inside an exit, not when everyone is inside the SAME exit. If everyone is in the hub exit when one person goes into the pipe dreams exit, everyone will be sent to pipe dreams. You don't really need to grab almond water during this part because, again, you won't be losing much sanity before getting to level 4. You could also dupe while waiting for the person doing the level.
Level 2-1 (Pre-Station): If one person isn't trusted to not die have another person wait at the darkness to do the bug where the smiler doesn't spawn (or at least have only them get chased).
Level 3: First, there is a way to completely negate sanity loss in this level. Anyone that is not getting breakers in the West Substation or Generator can simply go into a breaker in the East Substation. (You can enter breakers even after completing them.) You will not lose sanity when interacting with things like breakers so that way you don't just die of sanity with no way to drink almond water. Decide who is going to do the West Substation and Generator before anyone goes into either of those areas. Sending two people to generator does have the strength of it being easier to get the out in the open or danger hallway breakers but has the weakness of having to do double the amount of running between doors and having to use double the amount of sanity items.
Level 4: Get the hub door if you aren't duping and simply go to the exit. You can also drop your flashlight here for an extra inventory slot. You can have someone wait at the button so everyone can run past the cameras but this is slower because one person still has to go through the entire level after everyone else is at the exit.
Level 5-1 (Main Area): Get a moth jelly and have someone dupe it to get the keys. If you're not duping, get the 3 jellies as normal.
Level 5-2 (Beverly Rooms): Have one person guess the code while everyone else listens and looks for entities. If you're doing the level legit, then have one person go to the right side. Lean around corners before going around them. Hug the left wall, and when you find the skin stealer, alert everyone else (sorry solo nightmare players). By now, the hound is most likely gone, but if it's not, it's best to quickly have it chase someone so you don't get trapped. Now you want to run around the entire level so nobody else has to worry about entities and can do the level normally. Make sure you're the last one to go in the exit.
Level 5-3 (Boiler Rooms): Just like for the smilers you can do the Boiler Skip where one person goes to the pipe exit while everyone else waits in the stairs and dupes if possible.
Level 2-2 (Pre-Fun): The same as the first pipe dreams except you can also avoid the smiler chase if at least one person goes to the station exit and you do the exit glitch.
Level Fun: Before you do anything here, please take note of the extremely risky "Level Fun Skip". One person can use momentum and jump to go into multiple slides so that the exit triggers without everyone going in. However, there is a chance you get glitched and do not actually go into either slide, forcing the other person to get to the slide before you die of sanity so that once you die the exit still triggers and you just lose your items. Alternatively, they can just die if the exits triggered is one less than the players in the server. Anyway, here is the strategy for the actual level. Either decide who should aggro the partygoer or have one person go behind the table and aggro them so they get stuck and don't chase anyone. For the second part one person can crouch jump on top of the chairs behind the table and pop the balloon to freeze those partygoers. WAIT for everyone to get to the corner before the "run part" because after the partygoers are aggroed they will face the area you have to go through. Choose one person to aggro the partygoers and go under the table to the left and decide which order everyone should go in to run past those partygoers. The first one to run has a high chance of the partygoers aggroing earlier. Dupe anywhere in the level if necessary, and check sanity often because it only takes 2 minutes and 47 seconds to die of sanity.
Poolrooms: Before going into the exit to Level !, figure out which order everyone should go in. Each placement has its strengths and weaknesses.
Level !:
1st: You are the farthest away from the horde but have to react to all of the traps in the level.
2nd: You are far away from the horde but could get messed up by falling lights.
3rd: You are not that far away from the horde but the lights should have already settled into place.
4th: The lights have already settled into place but you are a lot closer to the horde.
There are separate triggers for the anti-cheat partygoers and the actual horde. The trigger for the anti cheat partygoers is right after the second door to the right, and the trigger for the horde is right after the doors with the entities inside them. Count down before running.
The End: Have one person (non-host) use an autoclicker on the fire alarm after getting to full sanity. This will make the scratcher move very slowly towards it and it won't be able to kill anyone. Just don't be slow at collecting the tapes so that the person autoclicking the fire alarm doesn't die of sanity. The lights turn off after 90 seconds, but it's not an issue if you autoclick the alarm.
Level 9223372036854775807: One person can climb the stairs while everyone else dupes. Have one person (preferably the host) prepare a 3 minute timer.

Level 94-2 (Clown): First, dupe again. You can get on top of the fence in this level (this has NO risk of dying, compared to standing still there which has a small risk) by crouch jumping on top of the table near the fence to the right side of the rollercoaster, crouch jumping on top of the fence, then crouch jumping on top of the invisible wall, crouching across the gap, and crouch jumping on top of another invisible wall. The only risk involved in this strategy is that if you fall inside the rollercoaster tracks you will be softlocked and the run will instantly be dead.
Level 6: If someone doesn't know the path instruct them to follow someone who does in part 1 and have someone who does know it tell them the color of their LiDAR dots in part 2. Crouch at all times on part 2 unless the wretch is far away or you need to manage your inventory.
Level 7: You can either fast swim (much faster but very slightly risky due to sanity) or use a boat (slow but no sanity loss). You will not lose sanity if you are in a boat or wearing a diving helmet. Just go 45 degrees to the left when exiting the cage for the swimming section.
Level 8: You only need to crouch twice (first moth and last moth of section 1). The rest of the deathmoth part you can just run. In section 2, wait so that nobody gets stuck behind the falling rocks and has to go to the second path (risky). When you make it to the skin stealer part, put moth jelly in your main hand and both pockets if you have any. Have one person scout for stealers, and if it is safe they will tell everyone else to come. If you get chased, eat moth jelly. The skin stealer is too tall to fit inside the airlock at the start but can fit into the airlock at the end. Eat moth jelly when going past the final gap before the airlock. There is also a way to skip the section with the skin stealers as well as skipping 0.11, but it requires some insane parkour. https://www.youtube.com/watch?v=F5y0Y-NdKXQ&t=1572s (this is right before the falling rocks)
Level 0.11: Have everyone meet up at the arrow with empty space on both sides (not the one right next to the wall). As soon as you go in the wooden door thing sanity drain starts and the bacteria spawns. Immediately chainsaw the door to the left and have everyone wait in the vent and door. Close the door so the bacteria doesn't get glitched. Now, wait for the bacteria to chase the people in the vent. Don't get too close to the front or the bacteria will kill you. If the bacteria goes right after leaving the people in the vent alone, have everyone run the opposite way to the lever next to the gate. If it goes the exact opposite direction of the vent, then crouch behind it. If it turns right into the small hallway, have everyone run to the lever next to the gate. If it goes into the large hallway, continue watching it. If it takes the first left, have everyone run to the lever next to the gate. If it goes right, also have everyone run to the lever next to the gate. However, if it takes the second left, have everyone hide as it will turn around very soon. Once it leaves the main path to the lever, have everyone run to the lever next to the gate. After everyone gets there, have one person (non-host) autoclick the lever next to the gate so it opens. Now just input the elevator code (415314) and move on to level 9.
Level 9: Use the same strategies for the houses that I mentioned in the escape level 9 achievement section. If you have to lead a wretch outside, go near the garage house and try to lead the wretch into one of the collision holes. For the hub house, if you can not jump out of the window after getting the computer, have someone else come to the hub house and tell you when the wretch is far away from the door and then RUN FOR YOUR LIFE out of the house. Here is a map of the level if you need it.
Abandoned Outpost: Have someone collect one almond concentrate and bring it back to the start. Dupe until you have four, and then go into the shower room and put them all in. Once all four concentrates have been put in, everyone should go into the room before the room with the shower except for one person who will stand behind the trap so the wretch will just walk in and you can shower and pickpocket it.
Level 10: Firstly, only one person needs to do the level. Everyone else can wait in the exit back to level 9 (just go the opposite way that you usually go) and you can do the exit glitch here. The facelings on the road don't really serve as much of a threat because they run into the field when a player sees them. When you get to the barn, go into the field to the right of the barn, then jump on the fence, and run to the right, take the first left, take the first right, and then go on the edge until you are at the arcade. You can dupe at the start of the level, but sanity loss here is very slow.
Level 3999: Just run to the elevator and put in the code.
Level 0.2: Traveling in groups of two or three might be safer for repelling smilers, but could be very dangerous if you encounter a skin stealer. Not everyone has to do the level. It might also be good to memorize the routes from the noclip floors to the doors and back to a noclip floor. Here is a map of the level (made by hacer/Carbon998)
Level 6.1: Just put in the code in the typewriter and go to the exit. If needed, get supplies or take a break before level !-!.
Level !-!: This level requires extreme practice and memory. It is recommended to practice this level for at least 30 minutes before attempting it in nightmare. Make sure that you assign someone to open the door that is chainsawed so that the door doesn't get opened and immediately closed again, causing the squad to die to the Clump.
Level 188: You can skip the level by either using the elevator glitch or the TV glitch.
The easiest strategy (probably) for each player count is:
1 player: Practice the elevator glitch and do it.
2 player: Host practices the elevator glitch, other person dies of sanity to trigger a die skip
3 player: Two people need to glitch out, so you would need a TV, or have two people die of sanity at the same time (not sure if this works or not)
4 player: Three people need to glitch out, so again a TV is forced, or you can have 3 people die of sanity simultaneously (also not sure if this works)
Level 37.2: The main part of this level is very safe so I don't need to explain this. For the clump areas, everyone except for one person will go into the exit. That one person will search for valves, and if they die, the exit will trigger before the run ends and the only thing that will be lost is the dead person's items. Valves will also save if you die.

Level Fun+: Do the sub-areas of Fun+ in order of hardest to easiest (firework/dark blue, glowstick/red, balloons/light blue, simon says/pink). For the firework section, stay under tables and get good at stunning Party Hosts with the fireworks. For the glowstick section, memorize the places where you need to drop the glowsticks and be very careful to avoid the partygoer. For the balloon section, go to the left side of spawn first and just pop every balloon you see. If you get chased, just run under a table. For the simon says section, get the tape and pop the correct balloons (the order is always green, yellow, blue, red, pink/purple, so the balloons are popped in the following order: GGYGYBGYBRGYBRP). If you are low on items you can reload the main part of fun+ for items.
Level 52: Memorize the path and just crouch the entire way through the level.
Level 55.1: Here is the easiest way to get each picture:
Picture 1: Take a picture of the "Who is here?" message (solo), a teammate (duo/trio) or three teammates (squad)
Picture 2: The location is in the screenshot below:
Picture 3: The location is the screenshot below:
Go here from the left side and then hide in a locker until you know where the bacteria is. When it's safe, go to the elevator.
Picture 4: Go on the elevator and let it go up for about 2 seconds. Then, wait for the bacteria to chase and get the picture. After that, continue going up so the bacteria leaves.
Picture 5: The location is the screenshot below:
The smaller circles are the elevator and the pink light.
Before going for the picture, wait in the elevator for a few minutes and wait for about 20 smilers to die, then go. After getting the picture, hide in a locker until you know where the skin-stealer is. When it's safe, go to the elevator.
Picture 6: Go on the elevator and let it go up for about 2 seconds. Then, wait for the stealer to chase and get the picture. After that, just go up to the exit and escape the backrooms.
One time while practicing 55.1 i saw all the entities respawn and go back to their starting point. I do not know how this happens and will update this if I learn any more about this. I do not think it's based on the amount of time you are in the level for, as when that happened it had only been a few minutes since I started the level. I tried AFKing in the elevator for ten minutes and nothing happened. This is either a glitch, or a feature to prevent smiler extinction.

Levels that all players involved need to practice:
- Level ! (memorising the level)
- Level 8 (memorising the route if you decide to do the cave skip)
- Level !-! (memorising the level)
Levels that at least one person needs to be good at:
- Level 3 (getting breakers in hound areas)
- Level 5-2 (guessing the code fast enough before entities get to the start of the level)
- Level 5-3 (getting to the exit)
- Level Fun (aggroing partygoers or possibly doing level fun skip)
- The End (autoclicking alarm)
- Level 8 (knowing the route and scouting in the skin-stealer section, if you decide not to do the cave skip)
- Level 0.11 (autoclicking lever)
- Level 9 (getting computers)
- Abandoned Outpost (getting the concentrate)
- Level 10 (getting to the exit)
- Level 0.2 (getting the levers)
- Level 188 (getting the TVs)
- Level Fun+ (getting the keys)
- Level 52 (memorising the route)
- Level 55.1 (getting the pictures)
Levels that all players need to do but aren't hard:
- Level 0
- Level 2
- Level 4
- Level 37
- Level 94
- Level 94-2 (Castle)
- Level 6
- Level 7
- Level 8 (just going through the level)
- Level 0.11 (just going through the level)
- Level 3999
- Level 37.2 (just going through the level)
Levels that only one person needs to do but aren't hard:
- Level 1-1 (opening the elevator)
- Level 1-2 (getting the keys)
- Level 1-3 (memorising the level and telling everyone when it's safe)
- The Hub
- Level 1-4
- Level 5-1
- Level 9223372036854775807
- Level 6.1
- Level 37.2 (valves in clump areas)
I would recommend practicing nightmare by doing solo nightmare runs. First, practice all of the levels in the first two sections until you can do them 5 times in a row. For longer levels, this is not necessary, but you just want to be confident with every level. Use a hard save to do runs on update 3 and 4 levels. I would recommend practicing until you can do levels 7 to 3999 in one run and levels 3999 to 55.1 in two runs. Now you can start doing actual nightmare runs from level 0. Make sure to keep track of deaths and near-deaths. If you don't get to level 7 within ten runs, practice the levels that you died on the most. Once you're confident with these levels, get another sample size of ten runs. Repeat this until you make it to level 7. If you want to skip the clump areas in solo nightmare, grab a flare gun here. Alternatively, you can get the hub door and then go back later. Wherever you die (if you don't just first try all of the update 3 and update 4 levels), figure out how you can avoid dying there again. When you think that you're consistent enough to beat 4 player nightmare (or when you beat solo nightmare) you can start finding players.

The time it takes to run out of sanity in each level means the amount of time it takes for you to see the sanity death hallucination if you start at full sanity and do not use sanity items.
To convert from normal to nightmare sanity loss, just divide the time by 1.5.
Sanity replenishing = You automatically get set to full sanity when entering the level
No sanity loss = Sanity never goes down in this level
Very Slow = >10 minutes to die in nightmare
Slow = 5-10 minutes to die in nightmare
Medium = 3-5 minutes to die in nightmare
Fast = 2-3 minutes to die in nightmare
Very Fast = <2 minutes to die in nightmare
Level Name | Sanity Rate | Time to run out of sanity in normal | Sanity bars per second in normal | Time to run out of sanity in nightmare | Sanity bars per second in nightmare
(?) = Approximation
Level 0 | No sanity loss | N/A | 0 | N/A | 0
Level 1 | Very slow | 16:40 (1000 seconds) | -0.001 | 11:07 (667 seconds) | -0.0015
The Hub | No sanity loss | N/A | 0 | N/A | 0
Level 2 | No sanity loss | N/A | 0 | N/A | 0
Level 3 | Slow | 8:20 (500 seconds) | -0.002 | 5:33 (333 seconds) | -0.003
Level 4 | No sanity loss | N/A | 0 | N/A | 0
Level 5 | Medium | 5:33 (333 seconds) | -0.003 | 3:42 (222 seconds) | -0.0045
Level Fun | Fast | 4:10 (250 seconds) | -0.004 | 2:47 (167 seconds) | -0.006
Level 37 | Sanity replenishing | N/A | Infinite | N/A | Infinite
Level ! | Slow | 8:20 (500 seconds) | -0.002 | 5:33 (333 seconds) | -0.003
The End | Slow | 8:20 (500 seconds) | -0.002 | 5:33 (333 seconds) | -0.003
Level 922 | No sanity loss | N/A | 0 | N/A | 0
Level 94 | Fast | 3:47 (227 seconds) | ~-0.0044 | 2:31 (151 seconds) | ~-0.0066 (?)
Level 94 Castle | No sanity loss | N/A | 0 | N/A | 0
Level 6 | Medium | 5:33 (333 seconds) | -0.003 | 3:42 (222 seconds) | -0.0045
Level 7 | Medium | 4:50 (290 seconds) | ~-0.0035 | 3:13 (193 seconds) | ~-0.0052 (?)
Level 8 | Slow | 6:05 (365 seconds) | ~-0.0027 | 4:03 (243 seconds) | ~-0.0041 (?)
Manila Room | No sanity loss | N/A | 0 | N/A | 0
Level 0.11 (Pre-Bacteria) | No sanity loss | N/A | 0 | N/A | 0
Level 0.11 (Levers) + Fake 188 | Medium | 4:50 (290 seconds) | ~-0.0035 | 3:13 (193 seconds) | ~-0.0052 (?)
Level 9 | 3:20 (200 seconds) | 0.005 | 2:13 (133 seconds) | 0.0075
Level 10 | Slow | 8:20 (500 seconds) | -0.002 | 5:33 (333 seconds) | -0.003
Level 3999 | No sanity loss | N/A | 0 | N/A | 0
Level 0.2 | Very slow | 25:00 (1500 seconds) | ~-0.0067 | 16:40 (1000 seconds) | -0.001
Level 6.1 | No sanity loss | N/A | 0 | N/A | 0
Level !-! | No sanity loss | N/A | 0 | N/A | 0
Level 188 | Slow | 8:20 (500 seconds) | -0.002 | 5:33 (333 seconds) | -0.003
Level 37.2 | Sanity replenishing | N/A | Infinite | N/A | Infinite
Level 37.2 Clump Zones | Slow | 8:20 (500 seconds) | -0.002 | 5:33 (333 seconds) | -0.003
Level Fun+ Hub | No sanity loss | N/A | 0 | N/A | 0
Level Fun+ Subsections | Very slow | 16:40 (1000 seconds) | -0.001 | 11:07 (667 seconds) | -0.0015 (?)
Level 52 | Medium | 5:33 (333 seconds) | -0.003 | 3:42 (222 seconds) | -0.0045 (?)
Level 55.1 | Slow | 8:20 (500 seconds) | -0.002 | 5:33 (333 seconds) | -0.003 (?)

Once you get the nightmare achievement, you can start preparing for the new update. When the update releases, all you really have to do are the new levels in singleplayer for those achievements, but if you want to do nightmare again, practice the new levels. Obviously I can't give advice on how to practice them because they don't exist yet, but once the new update comes out and I beat the game I will update the guide. The guide will also be updated every time a new strategy is found if it is useful in nightmare or singleplayer.
If you want, you can attempt some of the extra challenges that I've come up with.
Glitchless - Beat nightmare mode without using any glitches. (You could take this a step further and attempt a promotional run, which requires a full recording.)
Isolation - Beat solo nightmare.
Nightmare Master - Beat solo, duo, trio, AND squad nightmare.
Double Squad - Beat 8 player nightmare (cheats required). My PB on this is level 4...
Nightmare Grandmaster - Beat nightmare with 1, 2, 3, 4, 5, 6, 7, and 8 players.
Quadruple Squad - Beat 16 player nightmare (you can use cheat engine or sismenu to increase the player limit). However, since this is not officially supported, many players will be stuck in infinite loading screens in most levels.
Why? - Beat nightmare with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, and 16 players.
Double Nightmare - Beat nightmare mode twice in a row.
Completionist - Beat nightmare mode while also picking up at least 1 of each type of item and unlocking every hub door.
Ultra-Glitchless - Glitchless but you don't bhop either.
Hell++ Mode - Previous difficulty but on nightmare mode
Carry - Beat nightmare mode with someone who's never completed the game before so that their first game completion is in nightmare.
Non-nightmare challenges:
Hell Mode - Hard mode but you play completely intended, set all of your graphics settings to Ultra, and use minimum gamma.
Hell+ Mode - Previous difficulty but with head bobbing on, VHS effect on, FOV all the way down, and FPS capped at 30.
Consistency - Use the fastest pitfalls strategy (balancing and bhopping at the same time) and do it 10 times in a row without falling.
Speedrun - Get to the end of level 94 without hiding (one day cycle).
Actual speedrun - Do some speedruns and join the speedrunning community.
Supersonic - There is a glitch in level 94's castle that allows you to fly into the air. If you crouch jump on top of the rollercoaster and jump, you will get launched straight to the fence. If someone walks into you when you are on top of the rollercoaster, this will instead give you vertical speed. To complete this challenge, you need to fly so fast that you reach the barrier above the level that makes you despawn.
Minimalism - Beat the game while only picking up 21 items in total. (This is the minimum amount of items required to beat the game. You have to pick up a bug spray and a moth jelly in level 5. You can clip through the door to the elevator which means you only have to get one key. You have to memorize level 6 because you can't see most of the level. In level 7, you have to get a diving helmet. In the manila room, you have to get a chainsaw. Then you have to get four bottles of almond concentrate in the abandoned outpost. You also have to get from level 0.11 to level !-! without dying so you only have to pick up one chainsaw. Level Fun+ really screws up the challenge because you have to pick up 8 fireworks and 4 glowsticks. You also have to pick up the camera in 55.1.)
Minimalism 2 - Beat the game while only using your main hand inventory slot. You can only carry items in that slot. This is much easier than Minimalism because of level 6. If you want to take this a step further, you could try this in nightmare.
Hardcore - Get every achievement without dying.
Stealth - Beat the game without being chased (exceptions are level 2, Fun, !, 94 castle, Abandoned Outpost, 10, 0.2, !-!, Fun+). This is way harder than beating nightmare and might be impossible.