While you're going through horde (or the campaign), pick up frag grenades. Press
up on the D-pad to select them and tag a wall with them by pressing
. Wait for an enemy to run by and the grenade will detonate and kill them.
Try to place these in a high traffic area where they can't be spotted easily. For example: If enemies are coming through a doorway constantly on a horde map, place a grenade to the left or right of the door on your side. Otherwise, they may spot it and shoot the grenade.