An enemy engaged as a bullet shield is when you hold him as hostage and shoot over his shoulder. Just get close to an enemy and when the screen tells you to engage, HOLD and you may hold him. Now you just have to let him get shot and go down and you get 1 bullet shield. (When they go down it gives you a picture of a body with arms wide open on the left side) You can also run into and enemy while holding and it will engage him as a shield also. This can be boosted with a friend by having him shoot the bullet shield or be the bullet shield. Remember that when you grab a non-computer enemy, he will have the option to escape from your grip!