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'.