Instructions
Go to the top of Mt. Coronet after you defeat the Team Galactic boss to catch Dialgia. Dialgia is level 47 when you encounter it. Dialgia's Roar of Time attack is very powerful, so bring a lot of potions. Dialgia is a Steel and Water-type Pokemon, so use Fighting and Electric attacks.
Beat Dialgia to find three Legendary Psychic types: Azelf, Mesprit and Uxie. All three are level 50 Psychic types and all three sit on water tiles, so use Surf to get to them. Azelf is in Lake Valor, Mesprit is in Lake Verity and Uxie is in Lake Acuity.
Obtain the National Dex to catch three more Legendaries: Heatran, Cresselia and Giratina. Heatran is a level 70 Fire/Steel type and lives on Stark Mountain. Giratina is a level 70 Ghost/Dragon type and lives in Turnback Cave. Cresselia is a level 50 Psychic type and lives on Full Moon Island.
Put the Pokemon Regirock, Regice and Registeel on your team. Go to the temple north of Snowpoint City to find Regigas. If you have the three special Pokemon on your team, Regigas will wake up and battle you.