done on mission 2: awakening
be careful, this achievement is missable
the achievement sensitive part is near the very end. You will separate from Hicks and need to go to a terminal to refuel the skiff. After activating this terminal, the objective will be "Return to the Service Skiff and escape the Sulaco" (press
to check objective). This is where you need to be for the achievement.
pull out your motion tracker (its
if you forgot). Find and kill all the aliens in the area. There are @6 of them. The achievement will unlock the moment you kill the last one.
don't rush! if you move to far ahead, and the objective changes to "Release the docking clamps", the area with the aliens you must kill will be closed off and you must restart the level to try again.