If you want to boost this one, there is a way to kill a lot in the first chapter. After you have to use a clone for the first time, you'll enter in a big area to reach your hideout. This area is full of guards and it's possible to sneak and kill almost every one.
With the save system, just do a save when you first enter into the room, do the run and reload your save. Kills are cumulative across all saves.
Here is a video I made for a decent run in which I managed to kill 18 people in about 5 minutes :
If you manage to be more silent when you kill the last guard, you can remain unspotted and continue your killing spree: the one who spotted me, another "scientist" in the room with fire (enter by one of the side, not by the main door or he will spot you), a guard patrolling on the ground and another standing guard, 2 "scientists" accessible by a hole in a wall (wait until they finish their conversation, one of them will turn around and walk, silent kill the first one, follow the other and silent kill him) and finally a handworker and a patrolling guard in the last room in which there is your hideout passage.
Correctly done, you can kill 25 people in about 7-8 minutes if you better manage than me in this video. I did it a bunch of time but not when I recorded it
So, you can just do this 8 times to obtain the achievement
EDIT: By doing silent kills as I do in the video, you also progress towards
I did unlocked it during these kill runs :)