This should say "aliens" or "Xenos" instead of "enemies". This doesn't work on the missions where you fight other soldiers. (However those "gibs" count for challenges.) I unlocked this on mission 1 (Distress) after you cut Keyes down. The first room you encounter aliens, stay back at the door. Most of the aliens will go towards Keyes, line up a shot with your underslung grenade launcher ( on your pulse rifle) when 2 are close together. Restart checkpoint if you miss or it doesn't kill both before you reach the stairs on the other side of the room. Recruit difficulty.