Instructions
Work as team if playing in co-op mode. Quickly revive your teammates when possible, as each member is vital to the team and advancing into higher Zombie levels.
Rack up points quickly by letting the zombies break off part of the barrier, exposing their heads and bodies more fully. Then shoot your pistol three or four times at a zombie and finish it off with your knife.
Rebuild all barriers once you have killed the intruding zombies. This will slow down future zombies that try to come through that entry point.
Open doors one at a time and have team members each take a turn opening doors. Taking turns ensures that players do not exhaust all of their money opening doors for other members. When you have more money, you can purchase better weapons in advancing levels and stages.
Upgrade your weapons periodically to ensure you have a suitable gun that quickly kills the zombies. The quicker you kill the zombies, the more points you will rack up, resulting in reaching higher levels.
Keep your team together in co-op mode. Do not wander off from your group. Your team is more powerful unified rather than split apart.