To unlock General Corrosive for multiplayer, collect all of the speed chips in the game.

Beat story mode to unlock this feature.

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 the level 15 minutes, the old droid in the jail cell will talk to you, when the mils come to get you, hit them with you're arm until they start shooting at you, when they do this hide behind the old droid and the mils will destroy him instead of you and a cleaner should appear! Now use it against those Titans!