An alternative solution which you can do solo.
While playing through the campaign you can get this without much hassle.
Playing through Mission 09 - Hope In Hadley's, you will be tasked with making your way to Operations and saving Marines along the way. I personally obtained this achievement by killing an Alien that pounced on top of an AI controlled Marine. The exact Marine is fighting an Alien and you have to cut the door open to get to him. It's not hard to miss, it's right next to the first dog tag of the mission.