Instructions
Kill zombies. The entire point of the zombie mode is to kill zombies, which gives you points. The more zombies you kill, the more points you get.
Shoot zombies five or six times with your pistol, then go in with your knife. You get points by simply damaging zombies, and you get extra points by killing them with your knife. Do as much damage as you can before going in for the kill to get the maximum number of points for each zombie.
Allow zombies to break in through the barriers. Stand near a window and let two or three zombies break into the camping area. Kill them while they are inside the area and they may drop powerups, including a 2x Points powerup, which gives you double the points. Pick this up and go on a zombie killing spree while the powerup is in effect to collect as many points as you can.