Instructions
Beat "Campaign Mode" and watch all the credits to unlock zombie invasion mode. Once this is unlocked, go back to the main menu and select the new mode from the list.
Run over to the wall to purchase a weapon. Take cover behind one of the barricaded windows.
Crouch down and aim your gun forward. Once you spot a zombie, begin firing. After you kill the first few zombies, another wave begins.
Hold "A" button to rebarricade the windows. Reload your weapons while waiting for more zombies to spawn. When the next wave begins, resume your position before and continue shooting until the next wave.
Move to the locked door once the next wave begins and unlock. Crouch near the crate with the question mark on and purchase a weapon.
Continue firing to rack up more points. The zombie mode never ends, so continue to playing until you die or feel you have enough points.