Dishonored Definitive Edition

I followed this guy and it really helped me! Credit goes to "coopafication" for making this video.
EDIT: first video I posted was deleted or something from youtube so I post another good guide

Don't be discouraged. This achievement can be pretty luck based but you can still tilt the odds in your favor with a few tricks.

Dont worry too hard about conserving your ammo:
A dead run is dead anyway but a live run with no ammo is still a possibility.

1-2 combo:
Almost every enemy can be killed quickly by shooting them with either the gun or the crossbow and then attacking with your sword while they stagger

Shotgun approach:
The pistol has some spread which is useful for interrupting when your enemies rudely engage in combat without you

Whale oil dispenser:
There is a whale oil dispenser under a roof in the middle of the battlefield. If you're confident in your throws you can get a quick kill with it every once in a while

Drop Kill:
Jumping above most enemies (even when theyre aware of you and even from the same starting height) will allow you to drop kill them

Starting locations:
There are 8 spawn locations on the ground level. At the beginning of each round from the middle of the map you should have 4 within your sight and quickly reachable. Since only assassins spawn on the rooftops you should focus on the enemies on the ground to interrupt factions hostile to eachother. In round 10, 11, and 12, you should start on the shed above the whale oil dispenser since it offers a quick route to both tall boy spawn locations.

Bend Time:
Bend time should be the only power that you use, except single blinks to get onto certain rooftops. By round 7 if you havent used any powers you should have enough mana elixirs to use bend time once per round for the remaining rounds, and twice on round 12 for each tall boy.

Special spawns:
The music box overseer spawns in round 8 and 12 and will not usually get into fights with other factions so hes usually safe to leave for last.

The executioner spawns in round 9 and will fight every other faction so eliminating him first is important. If he spawns in an area with a ceiling try throwing down your spring razors and whaling on him while time is slowed.

Tall boys spawn in round 10 and 11 and 2 appear in round 12. They will kill everyone whether by accident or on purpose so killing them first is also important. Luckily they have a few drawbacks: they only spawn in 2 locations and they dont move very quick so lining up a drop kill is easier than on the quicker shorter foes. Use bend time as long as they remain unkilled and learn how to get above them quickly.

Round 12:
Round 12 is not the hardest round but if you get unlucky with spawns it can easily be the end of your run. 2 tallboys spawn in and should be dealt with quickly via bend time and drop kill. If the music box spawns near them and starts playing then the run is pretty much dead but if he doesnt you should have 2/5 enemies taken care of by the time bend time ends. Since the music box overseer doesnt fight others youre good to take care of the other two enemies before handling him. Basically once youre done with round 11 take a deep breath and pray for RNG and dont blame yourself if round 12 doesnt work out.

Restarting:
Once youre in the zone it can be very disheartening to restart and go through the slow early rounds. Playing through a dead run can help you understand future rounds and reset your patience for those first rounds.

Round breakdown
Round 1: 3 enemies (3 total)
Round 2: 3 enemies (6 total)
Round 3: 3 enemies (9 total)
Merchant (10 total)
Round 4: 4 enemies (14 total)
Round 5: 4 enemies (18 total)
Round 6: 4 enemies (22 total)
Merchant (23 total)
Round 7: 4 enemies (27 total)
Round 8: 4 enemies (31 total)
Round 9: 5 enemies (36 total)
Merchant (37 total)
Round 10: 5 enemies (42 total)
Round 11: 5 enemies (47 total)
Round 12: 5 enemies (52 total)
Merchant (53 total)