Hillbilly Doomsday (EP)

This is the first zombie, and he's super easy to defeat. Just stand and shoot at him and dodge his attacks. He rolls at you, just jump over him. He throws rocks at you too, but again these are very easy to dodge and don't do much damage. His other attack is shaking rocks down from the ceiling, again easy to dodge and low damage.

As long as you go into this boss with 3 hearts and keep shooting him, you could likely just tank it and take most all of the damage he does to you. He also drops health, which is always nice.