Styx: Shards of Darkness

Easiest level for this is the prologue. Use your vision to identify enemies and make noise to see if any more are in the vicinity.

By the end you should have 31 kills.

Best place I've found is at the start of Mission 2 (Korrangar).
Take the rope, jump down the first building and wait behind some crates at the cliffside. A patrol will come soon after and you can sneak up behind him and throw him off the cliff.

Save just before you push him and reload until the Achievement pops.