Play in Co-op mode. By the window at the back that you can see a truck out of, find a corner that is to the right of that window. 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 do not move. If a zombie comes in, knife him. Shooting will not always work.