Metal Arms: Glitch in the System Cheats

in multiplayer sometimes you can kill yourself and get bots to help you fight

When playing multiplayer, if you destroy enough enemies without getting
destroyed yourself you get a message saying "Brawling Bot Supreme". It
would be the second message you would receive not the first. If that
message shows up you will receive 3 robots that will help you defeat the
enemy. The message will show up after killing 6 enemy bots without
getting killed. This code only works if you play another person not the
CPU.

Reach the part of the They Live level where two zombie bots are trapped in two cages. Free them by cutting the red wires that are holding the cages and they will become your allies.

To get some allys on any of the levels were you are chasing the robot for the info you can find a creature that regenerates itself.
If you see them in a cage in the air use the disk gun cut the rope and they will destroy enemys for you and follow you until you die.

Beat the Story mode to unlock the concept movie.

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.

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