Right after a drop pod in Act 7 Ch 1 a quick response sequence of the ship flying over head will signal a timer to start. You'll have 2:00 to make it to the ramp, the easiest way by far to eliminate everyone on your way is using the charged shot from the Bone Duster because it kills everything in it's path regardless if an obstacle is in the way or not. No aiming, just from the hip in their general direction and you're good to go. So max out on them before starting your run!
At one point you'll come across a hallway with enemies encountering cross-fire from the ship, ignore everyone and sprint past them, the don't count towards the achievement.
Keep the killing up until you hit ground level, once you leash the ramp for the ship and have killed everyone the achievement will pop.