For the first few rounds, allow the zombies to break down the boards and come inside. Use your pistol or knife to kill them to gain more points than by shooting them once in the head. A knife to the head gives 130 points per kill. Once all of the zombies are dead, patch up the boards. You will have much more money this way than if you simply killed them with guns. Note: After level 3, the zombies are too strong to be taken down this way.