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. I personally liked the Penetrator because it's a one hit kill, but other powerful, quick killing guns like the Bone Duster are good too. Once you make it to the escape pod, the achievement will hit if you killed every enemy. Credit for the video goes to jhaller2