The last Legendary Pokémon is Regigigas who needs Registeel, Regirock, and Regice to be woken up. To get the first three Regis you will need to migrate them from Ruby and Sapphire through Pal Park
Catching Regigigas[edit]
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Snowpoint Temple 1F
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Snowpoint Temple B1F
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Snowpoint Temple B2F
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Snowpoint Temple B3F
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Snowpoint Temple B4F
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Snowpoint Temple B5F
Once you have the other three Regis, make sure you have them in your party as well, go to Snowpoint City and head inside the temple towards the top if the city. The temple has six levels with some of them having simple ice sliding puzzles to solve and wild Pokémon to face. There are a few items as well with a hidden Full Restore on the rock in the second top row of ice on B3F. On B5F is the big man Regigigas.
Regigigas is a lvl 70 normal type who knows Stomp, Confuse Ray, Superpower and Zen Headbutt. Simply put it to sleep and use a Fighting type(be careful of its Zen Headbutt as its a Psychic-type move) to lower its hp then use False Swipe to bring it to 1 HP and throw Poké Balls(recommended are Dusk Balls).