Have a Regigigas in a Cherish Ball (it does not have to be the event Regigigas, you can hack it) in your party, and go to the end of Iron Island. There's a small cave room that would have Registeel in it. Interact with the statue at the back of the room, and a Registeel level 30 will battle you. I'm pretty sure that if it faints it respawns immediately, but please correct me if I'm wrong.
