Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness/Chapter 9: The Mystery of Fogbound Lake

Boss 3: Groudon Illusion
Start the same as your last boss battle: Seeds and Disabling moves. Once Groudon is under the influence of one of your skills or seeds, begin attacking. Because of Groudon's Ground Type, Grass and Water work really well here. If not, just use your strongest attacks. Don't forget those juicy Oran Berries!