This acheviement should come with natural progression, and I unlocked it before the end of the first mission.
That said, I understand you can make use of the checkpoint system to boost this if you want - this best place I have seen is towards the end of the first level, when you enter the Cargo hangar to let in the drop ship. When you first enter the room, a xeno will pop the grate on the floor in front of you and crawl out - relatively straightforward shotgun kill. I know this one counts as my acheivement unlocked after i blew him away. So if you really want to boost the cheivo, just make the kill, then restart the checkpoint.