This can be done in a co-op mission or in multiplayer. In co-op, you can easily have this by the middle of the second mission. The first mission, an alien can attack your partner and start a grab animation (where they're about to use their second mouth to bite them). Shoot the alien point blank with the shotgun for the achievement. In the 2nd mission, you can shoot a facehugger while your friend is pressing from a close encounter for the cheevo as well. In multiplayer, when a teammate is pinned by a lurker, shoot or melee it before they're killed for the achievement.