In Order to farm money, let zombies live, and let them caress the window, it wil consider that zombies instantly destroy planks you try to put on them, but if you do try to put planks, you will still get the money from the installation, wich means than you can spam planks in order to make 100 every 1,5 approximatly.

What the glitch does :
if your note moving your character, and shot at an angle or on a specific surface that makes some sort of ricochet effect, the game will try to calculate how big of an explosion it really was, and goes on forever if you dont cancel it by moving your character, using this, you can basicly clean up the entire wave because for some reason, while the glitch is active, money goes up, and all zombies start to slowly die away, you cant really see much cause your camera is going insane, so im assuming its either that the explosion keeps on going and zombies keep coming towards you, or that the explosion kinda reverbs itself across the map.