Aliens Colonial Marines

This achievement is for clearing Mission 9 (not to be confused with Mission 4, No Hope in Hadley's). For the final part of the mission you will need to disconnect 3 fuel lines to allow the dropship to take off. Initially I was struggling due to the sheer numbers of aliens that were appearing. However they don't just infinitely respawn, so the key is to take your time. When you first enter the area grab ammo and armour as needed, but don't touch the fuel lines. A number of aliens will spawn on the roof of the large building you came from, so stay near the ship and you should be able to take them out at a distance.

Now you will need to disconnect the 3 fuel lines, and deal with the wave of Xenos that spawn after each one. It's very easy to be outflanked and blindsided here, so by far the easiest solution is to run back into the large building you came from after dealing with each wave. This means most of the aliens will go after the other Marines, giving you room to breathe. Just be careful not to stand too close to the door, as a few may spawn from the roof and drop down near you. Once the coast is clear go for the next one (you are actually invincible when disconnecting one, but it's not a good idea to try it with any Xenos about as they will simply queue up behind you)

As soon as all three have been done the dropship begin will take off, however to end the level you do not have to board or be close to it. So run back to the building, let you teammates do the leg work and wait for the cutscene and achievement to pop