When you get to Ratchet's huge robots with big blue eyes, try to destroy as many of them as you can. Wait until one of the robots' eyes open up and turn blue. Shoot the blue eye before it closes back up with one shot. The next part is much more difficult; you must battle Ratchet on top of his machines. Be careful; he can do three moves: order his robots to attack you, throw a bomb at you (which you can shoot to destroy), and jump from one robot to another. When he stops on a robot, shoot him quickly. Do this process a few more times and he will be defeated.