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.
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.
Successfully complete the game in campaign mode.
Successfully complete the game with all secret and speed chips to unlock

General Corrosive as a playable character in multi-player mode.
Beat story mode to unlock this feature.