Welcome!

This guide is based off my own experience, testing suggestions from forums, and discussions from the Reddit Dishonored Discord group. Others have copied this guide so that's proof my tips work.

First, the main tips. I recommend reading these, then trying again. Then, if you're really struggling, see below for weapon & strategy for each round.

If you have suggestions, please comment and I will include them with full credit to you.

• Check your difficulty level in-game
  While there's no difficulty level in the challenge, you should go back to your most recent save. The By My Hand Alone achievement is still very challenging even if you set the difficulty to easy.
  
  
• Power management
  Save your power for both single Blinks (let them self-replenish), and for Bend Time at the beginning of rounds with TallBoys.
  
  
• Stop counting
  See below for a table I made to simplify everything you need to know. I recommend that you have this open on your phone or 2nd monitor. This way, you don't need to memorise how many kills you need or who spawns in or when you can expect a specific power.
  
  
• Crossbow + Aim assist
  If you haven't already turned this on, go to Settings to use aim assist. Crossbows allow you to headshot accurately, which is invaluable for assassins or enemies that aren't close in the last few rounds. Each assassin offers 2x bolts so always rob their corpses. :)
  
  
• Tallboys must die first
  They're equipped with a long-range one hit kill and can destroy your perfect run in seconds.
  
  
• Only use Bend Time AFTER everyone has spawned in
  
  
• Start every Tallboy round - Round 10, 11, 12 on top of the shed above the Whale Oil Tank Dispenser.
  This allows you to: Bend Time as soon as they spawn, sprint over and drop assassinate.
  On level 12, with two tallboys, you will need to Bend Time twice - after the first dropkill, return to that shed to get the drop on both (start with the one to your right, as this is slightly harder to do as you need to leap from the vent).
  
  
• Luck.
  Yes - this is still a luck based achievement, spawn locations can ruin your run. Don't lose hope - you WILL get this! Blame random bad luck and keep trying :)

What numbers to check you have *after* the round + any noteworthy opponents.

Kill totals (+merchants), what type of enemy to expect, and which power you'll receive at the end of each round.

Short version:
tl;dr: Play aggressively, use your sword EVERY opportunity you can, if you can't, then crossbow headshot (use autoaim), and if you can't headshot, use bend time and dropkill.

Long version:

Part of the challenge with BMHA is resource management it may be worth knowing what to use against who.

Comments/questions/suggestions let me know.

• Sword
  You have likely worked this out already, but ALWAYS use your sword:
  - For the last enemy of the round.
  There's no reason to use anything special unless it's the Overseer with the Music Box, and even then, ideally you'd hide then drop assassinate.
  - On every single enemy if possible for Rounds 1-3, and even better, for Rounds 1-6
  - On ANY nearby enemy
  - Drop assassinate Tallboys (it's also possible to sneak up, double jump, and perform an assassination, however I don't recommend making that your focus as it's hard to be consistent about compared to dropkill)
  - Drop assassinate the Torturer. Don't engage in sword fight, just jump from a height or double jump that big boy, problem solved.
  
  
• Crossbow Bolts
  From round 7, assassins will spawn and has 2 crossbow bolts, for a maximum of 10 (until you get the expanded bolt pouch in one of the last rounds).
  - This is the most accurate, one hit kill long-range weapon
  - Excellent for any enemy except Tallboys (for whom drop assassinate are best unless you're able to sneak up, double jump and assassinate)
  - If you're not great at headshots, you can cheese this by turning on Auto-Aim in the settings menu.
  - Assuming you're robbing every assassin and using your sword mainly, you will not have a problem with ammunition.
  - You can also try going for the legs of Overseers with Music Boxes with your crossbow. This will take two hits to kill however.
  
  
• A Single Blink
  The aim here is that this will get you to where you need to be ASAP, however, by ONLY using it once it will regenerate, and will not affect your mana when you need it later in the game.
  Mobility is critical - always get yourself to the thick of the fight asap.
  
  
• Quick Access wheel
  This is a cheesy suggestion, and on first inspection this may be a weird thing to include but, it *almost* stops time which is great for when you need to think about the next best position for yourself. It also is partly transparent which gives you time to inspect your surroundings.
  Useful for when you're on Round 12 and you've got a perfect run so far and you need to calm the eff down :)

There's nothing wrong with these, they just may have some drawbacks - but you know your playstyle best.

• Bend Time
  The main thing with Bend Time is: only use after everyone has spawned in, or when you see the Tallboy spawn in.
  I've seen some advice guides recommending to use this as soon as you get it at the start of Round 6 to help with Asssassins, however, you CAN hold onto it until round 8 if you really want to cheese Rounds 10-12 with the Tallboys.
  I suggest Round 8 because locating & killing the Overseer with Music Box can make or break a run.
  For Round 12, use Bend Time at least twice. This will give you enough time to get across the map stealthily to first dropkill both Tallboys, then take out the Overseer with Music box.
  
  
• Grenades & Spring Razors
  Both are great for dealing with the Overseer with Music Box, if you're unable to drop assassinate. Otherwise, I don't recommend relying on grenades unless you're a good shot, though spring razors can be put down where you know enemies will spawn for basically a free kill (though, save this strategy for Rounds 11 and 12 if you can).
  
  
• Pistol
  I recommend avoiding this unless you're within range that will one hit kill, it's a little slower than the crossbow and less accurate broadly. That said, not a bad option for dealing with dogs or when you can hit two enemies at once - this will have enough stopping power to prevent them attacking, and give you time to drop assassinate both if you want.

Please note: these are obviously very effective if you're playing Backalley Brawl for fun or for points, but if you're trying to get BMHA I broadly advise not reaching for them, or reaching for them sparingly.

• Devouring Swarm
  Uses up more mana than you will regenerate, which is power you need for Bend Time when Tallboys spawn in Round 10, 11 &12.
  
• Bloodthirst
  Only mentioning this because I don't recommend this being your focus! You only get it in Round 11, so in Round 12 if you're going for this achievement this won't really change your playstyle. Otherwise, it's fun for Round 13.
  
• Windblast
  See reasons for Devouring Swarm. Otherwise, this isn't lethal quickly (unless you're in a really specific situation), so shouldn't be what you reach for.
  
• Returning thrown grenades to Overseers
  This is a fun option if they're the last enemy left, otherwise, most enemies are dodging these easily anyway so don't let hubris get in your way of a good run. Unless you're better at throwing them accurately than I am, in which case, gold star for you! Go for it.
  

If you have other strategies, please let me know!

Here are some other things you can try:

• Load up a regular game of Dishonored, and change the settings to easiest. This will somewhat reduce difficulty. I haven't recently verified this one but I've seen a few people mention it helped.
  
• You can also try creating a pile of Whale Oil Tanks where the Tallboys spawn, with the intention of putting 8 whale oil tanks down before Round 12. To do this, you will need to keep one person alive at the end of a round and wait for whale oil tanks to keep spawning. Personally, I've never been patient enough to get this to work so if you've tried this, let me know in the comments.