How do you get groundon on Pokepark adventure?

You can't get Groudon in Pokémon Ranger: Shadows of Almia, the game you're likely referring to when you say "Pokepark Adventure." Groudon is a legendary Pokémon and is not found in this game.

It's possible you're thinking of a different game, such as:

* Pokémon Ranger: Guardian Signs: This game has a different set of legendary Pokémon.

* Pokémon Mystery Dungeon: Explorers of Time, Darkness, and Sky: Groudon is a rare encounter in the post-game of these games.

* Pokémon Diamond, Pearl, and Platinum: Groudon is a legendary Pokémon found in the game's post-game.

If you can clarify which game you're talking about, I can give you more specific information on how to get Groudon.