I found it easy. to get through the levels, just stand next to the enemy spawner and keep swinging your pipe. you never get hurt, just rack up ammo you dont need...
I am extremely confused by the fact that walls of death and floors which you can drive on in reverse gravity are the exact same in looks. it annoys me. make them different.
 "faze"){kind=icon}
 "Thing-Thing 4"){kind=icon}
 "Blackboard Squash"){kind=icon}
 "The Visitor"){kind=icon}
 "Shopping Cart Hero"){kind=icon}
 "Ragdoll Invaders"){kind=icon}
 "Rocket Car 2"){kind=icon}
 "Bars of Black and White"){kind=icon}