To battle Regigigas, you need Regirock, Regice, & Registeel in your party (which means that you have to transfer them over via Pal Park first). Once you do that, you can enter the Snowpoint Temple and go through a few ice puzzles to reach Regigigas. It will be at Level 70, but has the trait Slow Start, cutting its Attack & Speed in half for a few turns, so use that to your advantage. If you don't have the other Regis then Regigigas will still be there, but it will be asleep and unfightable.