For the first few rounds, let all of the zombies break down the boards and come inside. Then, use your pistol or knife to kill them to gain more points than simply shooting them once in the head. A knife to the head will earn 130 points per kill. Once all of the zombies are dead, patch up the boards. You will have much more money than if you simply killed them as they approached. Note: After level 3, the zombies are too powerful to be taken down this way.