During the first level, after having grown up, go to where Rodney's dad is located. Once you are in the diner in the room where he is found, he asks you to select the Scrap Launcher. Select it, but do not shoot targets yet. Instead, shoot everywhere else, but not at the targets. Use up all your scrap, and Rodney's dad will give you 10 extra scrap.
When you reach Ratchet's huge robots with big blue eyes, try tdestroying 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 tougher. 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 fast. Do this process a few more times and he will be defeated.
When you fight the Sewer Bot, he uses two attacks. He hits a cannon and fires one or two sewer water balls; and fires one or two robot dogs from a separate cannon. Note: Once the Sewer Bot has shot from all, or a couple cannons, he will have to reactivate them. However, Rodney's cannons reactivate automatically after awhile. When the sewer bot is running to a cannon to shoot it, hit the cannon facing that cannon and hit it a few seconds earlier. Otherwise, he will move away to another cannon if you shoot it too late, and he will not have gotten there yet and the sewer water ball will hit the ground. Time it correctly for him to get hit. Note: He will usually fire several robot dogs over to destroy you if you hit him. Keep hitting him the same way a few more times and he should be defeated.