Collect the corresponding number of secret chips during campaign mode to unlock the corresponding multi-player level.

Corrosive City: 110 secret chips
DMA Death Sphere: 40 secret chips
Mac mines: 138 secret chips
Matt's Big Gun Fun: 8 secret chips
Morbot Land: 15 secret chips
Ruins: 60 secret chips
The Coliseum: 85 secret chips
The Reactor Factor: 25 secret chips
The Trenches: 4 secret chips
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Defeating General Corrosive
In last level, you will find yourself in ruins of the Space Station that you destroyed earlier. After General Corrosive survives the crash, he will get very mad, but will not be that "Indestructible". First, try to shoot off his right leg so he will not be that fast. Then, go for his left arm that he uses to get those flying green things out of his mouth. Next, go for the whole thing. It will be easy with rockets and few "cleaners".