Fighting the Zombiebot King
In order to beat the Zombiebot King just wear him down using rockets until he pauses (doesn't move). Toss a grenade (coring charge) into its mouth. On Normal, it takes five grenades to kill him and after the 3rd and 4th grenades there are regular zombiebots that swarm the arena.

Multi-player: Invincibility
In any multi-player level, press Start. You can be shot at, but receive no damage. You cannot move, but it is better than dying.

Original Concept FMV sequence
Successfully complete the game in campaign mode.

Multi-player General Corrosive
Successfully complete the game with all secret and speed chips to unlock General Corrosive as a playable character in multi-player mode.