This game’s levels are divided up into 5 phases. Each level has three challenges: reach the exit, collect all brains, and a time challenge. Once a level is completed, it will score the attempt with a star for each challenge met. To 100% the game, obtain a 3 star rating on all levels within normal and nightmare difficulty.

Each level starts off in a strategy mode, where you can view the level map and plan out an escape. Take note of the location of obstacles, switches, character warps, and of course the exit. At the top of the HUD it indicates how many resources you have (undead recruits of various abilities.)

After looking through the map in strategy mode, click on the starting location, choose one of the recruits, and pick a direction for them to go. This will switch to arcade mode, where the character will begin moving through the map. You must now guide them using the left mouse button to perform their action. That is all the control you have over them. Once they reach a warp or die or right-click the mouse button, the level will switch back to strategy mode and you make your next move. Repeat the process until you reach the exit.

Different Deadlings have their own unique abilities:
Bonesack
Automated: Running movement
Action: Jump
Other: He will die if he runs into a wall, unless there is a spring on it

Creep
Automated: Climbs on walls, ceilings, and floors
Action: High jump/leap

Stencher
Automated: Running movement
Action: Float
Other: He will change direction on contact with a wall

Lazybrain
Automated: Stand still
Action: Walk.
Other: Note that he cannot jump. Usually the level will have platforms to account for this.

Normal Mode:

Normal Mode:

Normal Mode:

Normal Mode:

As long as you unlock the 3 challenges in all levels, including the bonus levels, most of the achievements should unlock. The descriptions on most are self-explanatory. Here are any that may need additional work/explanation:

Complete level 04 using one zombie.
I was able to do this using the sticky guy. See the Phase1 video for how I did it.

Complete level 14 in less than 10 seconds.
Don't go for the switch. Instead use sticky dude going toward the right. When he gets to the edge of the starting platform jump and he should make to next platform up toward the right. From here jump to the wall and then to the exit.
Here is a video on how to get it:
https://www.youtube.com/watch?v=lmULlqOXBB0
Kill the zombie with the other zombie.
I got this on level 50 when a runner and climber collide into each other.
Here is a video on how to get it:
https://www.youtube.com/watch?v=4gTubjyVwZA

It’s very difficult to control the flying guy for long periods of time as he tends to be very bouncy. I found it useful to make a script that toggles his action on/off while held down to stabilize his flight so he continues to drift at the current angle or stay straight. This can be done with a keyboard macro. If you are not familiar with one, I used a freeware tool Autohotkey: http://www.autohotkey.com/
Below is the script I used and can be changed from the “right arrow” key to whatever key you have bound to the slide action. Just copy the code and save it to a text file with an .ahk extension. Double-clicking the file will automatically open and run the script in Autohotkey.