Play on Nacht Der Untoten in Co-op mode. By the window at the back where a truck can be seen out of, find a corner that is to the right of that window and have one player go there and crouch down. Have the other player jump on top of him, then have the player at the bottom stand up. You will now be inside of each other and are invincible as long as you remain immobile. If a zombie comes in, knife him. Shooting doesn't always work.