When he got hit the first time, some parts broke off his car/boat//tank/thing. They're over to the right. Use those to finish the magnet-ladder, and climb it as Robin. When you're on the sign, smash the thing in the center, and then use the magnet to remove the screw-looking things.
Member of the Super Ethical Genki Hunters [Saints Row the Third]
Is that the one in the parking lot? If so, On the right side of the screen you should be able to uncover some pipes and Robin needs his magnet outfit to climb up and over Joker and drop something on him. I think that is right.
Hall & Oates got guard dogs. Well, Oates couldn't afford them so he put an alarm company sign in his front yard. - The Bob