How to Unlock Regigigas
Like most Legendaries, Regigigas is only available in the post-game so make sure you’ve completed the main missions. You do not, however, need to have completed the Poké Dex to catch it.
Cogita can be found in the Ancient Retreat during post-game and has a long list of Legendary-related requests for you to choose from. The one specific to Regigigas is Research 24: The Plate of Snowpoint Temple.
After that, it can be found in Snowpoint Temple, but it’s not quite as simple as going straight there. First, you’ll need to have the Stone, Icicle, and Iron plates.
Prior to reaching this point in the story, you should already have them. The Vespiqueen you ran into in an earlier mission drops the Stone Plane. The Frenzied Hisuian Avalugg will have given you the Icicle Plate. As for the Iron Plane, Heatran rewards that one.
Once you have them all, you can insert them in the temple and enter the room with Regigigas.
How to Beat Regigigas
Regigigas is a Legendary and you will face off with it when it’s Level 70. Unlike some of the other Legendaries, the fight is pretty straight forward and you’ll head into it just like any regular Pokémon battle.
The fight itself can be quite tough due to its level but it you’ve reached this point in the game, you should have some powerful Pokémon behind you.
The best Pokémon you could possibly put up against Regigigas is a Fighting-type. Since Regigigas is Normal, Fighting-type moves will be super effective and can quickly whittle down its health. You shouldn’t have too worry too much about a one-hit KO since its level is so high though as its health gets down further, be careful.
Status moves can also be helpful in making Regigigas easier to catch. Poison and burning can be a little dangerous, as if it takes more than a few throws, its health might deplete so much that it faints — and there’s no taking those status moves back!
However, sleep and paralysis are great for this, so if you have a Pokémon with Hypnosis or Thunder Wave, utilize them right before you start throwing Balls.
You’ll likely need a good amount of Ultra Balls to catch it and when its health is down, it’s really luck of the draw. Make sure you have enough Pokémon for it to attack as you keep throwing balls until one finally sticks. At this high level, it might take a few — or you might be very fortunate and catch it the first time you try.
Once you defeat Regigigas, you’ll get the Blank Plate which is essentially the Normal-type plate and a cut scene will play to end the quest.