In Act 7, Chapter 2, at the very end of the level a timer will start from 50 seconds. It's from that point where you need to kill every enemy up to the escape pod (7 enemies I believe). I personally liked the Bone Duster as you can get 1 shot kills. Once you make it to the escape pod, the achievement will hit if you killed every enemy.
Note: Do not start killing enemies until you get to the top of the ramp and the timer starts.
Happy hunting