There is a clitch on zombie level,need 2 people up stairs one near the window where the cupboard with the sniper rifle is to stop zombies coming in window, you jump inbetween a wall and lie down it forms a invisible bubble that the zombies cant get to you,but nobody can walk in front of you while in the bubble or it breaks,make sure you walk behind
if 2 other players stay in the other room with the extra grenades on wall you can get realy faR