Instructions
Use your knife in the beginning levels. You'll save on ammunition and get more points for killing with a knife than using a pistol or rifle. Your knife should be adequate to get you to level four without overwhelming you. Avoid purchasing guns off of the wall in the beginning levels. Instead, save your points to open doors and pay for better weapons.
Assign a window to each person at the beginning. It's easier to manage a single window than it is to watch all angles as zombies attack. Repair the window after zombies get inside to gain points and prevent them from re-entering quickly.
Purchase perks as soon as you encounter them, especially quick revive. It's essential to be able to bring your teammates back to their feet quickly if a zombie knocks them down. Juggernog gives you more health, allowing you to handle more contact with zombies without going down. Faster reloads allow you to keep groups of zombies from knocking you down.
Open doors as a collective decision with your teammates. Confer with each other through text or use a microphone and headset for faster response times. Share the responsibility of opening doors to keep the point levels balanced, without putting one player at a disadvantage.
Gather together when the hellhounds approach, typically on level five and continuing every four to five levels. Back up against a wall and fend off the dogs as a group.
Utilize the mystery box to get a weapon instead of selecting one from the wall. The ray gun is deadly and highly effective at killing zombies as they rush to attack you.