Get any grade on all levels on the Child's Play difficulty setting to unlock Andy's Surfing costume.
When you see the intermission sequence with Fat Zombie, immediately run to the left side of the downed power line and the mini fence. Once there, you should find a trampoline. Jump on it and stay on the houses. The exploding zombies will not be able to hit you, but will damage Fat Zombie. When you see Fat Zombie jump, run to the right onto the taller house, and you will not be dazed. Keep hitting him with your nailguns and throw bombs at the tree so that they will bounce back at Fat Zombie until he has been defeated. This is also helpful in Horror, when you fight two Fat Zombies.