you need regice, regirock, and registeel in your team when you fight him
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mewtwo has to be with you
To awaken Regigigas you need to migrate from Ruby/Sapphire/Emerald the three Regis.
Pokemon diamond
You don't awaken him until you beat the elite 4. Then go to the weather institute and they will tell you where to go for each Pokemon KYOGRE/GROUDON.
you cant be a Pokemon in Pokemon diamond