cheat: after u buy the auto turret and rest for a day choose kill some zombies then place the auto turret outside and it will get the kills for u while ur in the safe zone
i call these the jesus cans: in the supermarket by the ladder there is trash cans if u start to climb the ladder and get to the top of the TRASH CANS not the ladder the u just walk on top of the cans and nothing can touch u and u can kill averything but the half zombies with a katana