Run around the map and only shoot zombies when they can hit you. This can be done with the pistol. The zombies will not get more health, as in Defense mode. Four shots with the pistol is enough to take one down. After a certain number of zombies appear, no more will spawn. When the zombies are following you in a large crowd all of them will want to travel in the same direction directly to you. This will allow you to simply kite them around the map. Take caution if there is one behind you. It will be better to shoot him when he gets in a dangerous position where it can hit you. It is better not to kill them; when a zombie is killed, another one will spawn somewhere on the map which also makes things dangerous. Kite them as long as needed to earn "Beginner", "Survivor", or "Veteran".