To defeat Metallo the last time, enable the "Unlimited city health" code. Pick up something and throw it at him. Make sure you are behind him or you will get hit. Repeat this until he is defeated. He then will shoot a missile and you have to ram it. Use Hyperspeed to ram it and you will have to fight Metallo again. This time he will use a beam to hurt you, and you must have to tap the buttons that appear on screen.