In Level 1: Distress, shortly after fighting the first group of aliens, you will reach the 
hangar bay. Your comrade will throw a grenade and destroy a dropship. Then, you will have to 
defend him until he opens a door. Two AI controlled allies are standing behind a barricade. 
None of the enemies can get behind this barricade to get the achievement. It is recommended to 
play on the Recruit difficulty. Stay in front of the barricade the entire time. Enemies are much
more likely to attack real players than AI controlled ones. You may take a few hits, but the 
shotgun will kill all aliens quickly. Once the door is open, immediately fall back.