How to make Instant Kills: Zombie: Get a Desert Eagle and shoot the chest.
Spider: Shoot at the eyes, if it is a spitter, aim at the acid bag. Robot: Get a magnum and shoot the head, recommended for Missle Robots
Hey, you should add to your game special zombies, some sort of The Last Stand: Union City, and maybe even a zombie as a Tank in Left 4 Dead 2, and the like, another cool thing would be to use their wood to take into a mission and barricading the entrances to the infected.