Where do you get Groudon in Pokemon Mystery Dungeon Explores of Sky?

You cannot catch Groudon in Pokemon Mystery Dungeon: Explorers of Sky.

Groudon is a legendary Pokemon and is not available in the main story of Explorers of Sky. This game focuses on a different set of legendary Pokemon.

While there are ways to obtain some legendary Pokemon through events or special methods, Groudon isn't one of them for this specific game.