To get regigigas, you have to migrate the 3 regis (regirock, regice, and registeel) from
ruby, sapphire, or emerald. then you have to go to the snowpoint temple WITH the regis.
go to the last floor and solve the ice puzzle. talk to the big pokemon. it's regigigas.
NOTE:you MUST have the regis with you or else it'll just say it's a statue.