When you crash on the level Outskirts step outside and you will see a grunt, kill him. Now look up to you right and left side. You will see two lights. Jump on one and grenade or crouch jump(grenade jump is a little hard on a light)up on the roof. Look around and you will see a small dark pathway. Walk down ithe path and you will find a skull and a couple frags. Pick up the skull and your HUD will disapear. Do this on any difficulty.