Styx: Master of Shadows

You will unlock this by simply progressing through the game, I unlocked it during the third mission - Deliverance. You can kill enemies by sneaking up on them and pressing or holding , poison food/water, drop chandeliers/crates on them, throwing knives, or by entering a duel.

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 :)

You will get some kills while completing the game. To boost the rest, right at the start of Conflagration Act 1 is an easy way to get the rest you need, four at a time.

To quote the walkthrough:
From the start, go to the left diagonally and jump across to the rusty pipes around the large structure (the furnace shaft). Drop down to the lower level and go through the door on the left to reach a familiar room. This is where you got the relic on a previous mission. Go right to the lever and send a clone out to lure the four Roabies into the room across from the lever. Wait until all four are in the room with the clone and detonate him, quickly hitting the lever to trap them in there and burn them to death.

Save immediately before you hit the lever and then you can hit the lever, reload, hit the lever, reload, etc until it pops, getting four kills every 15-20 seconds.