Things You'll Need
Instructions
Unlocking Pal Park
Proceed through the game and defeat the Elite Four and the Pokemon League Champion.
Complete the Sinnoh Pokedex by seeing every Pokemon in the region.You do not have to capture them, merely encounter them in a wild battle or trainer battle.
Go to Sandgem Town and speak to Professor Rowan to receive the National Dex, and at this point you can enter Pal Park on Route 221. Go to Pal Park and save the game.
Obtaining Groudon in "Pokemon Ruby"
Proceed through the game until you reach Route 121 just west of Lilycove City. Surf south from Route 121 and enter Mt. Pyre. Go left upon entering to access the outside of the mountain. Climb to the summit and defeat the Team Magma agents. Speak to the old woman to obtain the Red Orb.
Go to Slateport City and speak to Captain Stern. While you are speaking to him, Team Magma will steal the submarine in the harbor and flee. Go to Lilycove and enter the Team Magma Hideout just east of the city. Defeat the Administrator, which causes Team Magma to escape in the submarine during the battle, and proceed east of the hideout to Mossdeep.
Defeat the Mossdeep Gym to earn the Mind Badge. Obtain the Dive HM from Steven in one of the houses on the west side of the island. Assemble a party with access to Surf, Dive, Rock Smash and Strength. Surf south from Mossdeep until you encounter a dark patch of water. Use Dive to enter the undersea trench and follow it south until you find a cave. Enter the cave and surface into the Seafloor Cavern.
Explore the Seafloor Cavern and defeat Maxie. He will awaken Groudon, who will promptly flee and begin baking the world in intense sunlight. Go to Sootopolis City and speak to Steven to gain entry to the Cave of Origin. Head through the cave until you reach the steam-shrouded lowest floor, save your game, encounter Groudon (level 45) and capture it. Reset the game and try again if you accidentally defeat it.
Obtaining Groudon in "Pokemon Emerald"
Defeat Team Aqua at Mt. Pyre and obtain the Magma Emblem from the old woman. Go to Jagged Pass and enter the Team Magma hideout. Defeat Maxie to awaken Groudon, who promptly flees.
Go to the Aqua Hideout just east of Lilycove and talk to the guards to learn of Archie's location. Go to Slateport City and speak to Captain Stern to trigger the submarine theft. Return to the Aqua Hideout and conquer it to make Team Aqua flee.
Go to Mossdeep and defeat the Gym to earn the Mind Badge. Go to the Space Center afterward and fight alongside Steven to defeat Team Magma. After defeating Maxie, Steven will give you the Dive HM. Assemble a team with Surf, Dive, Rock Smash and Strength and head south of Mossdeep until you find the undersea trench. Dive into it and follow it south until you find a cave and surface there to enter the Seafloor Cavern.
Defeat Archie at the end of the Seafloor Cavern to awaken Kyogre, which causes a catastrophe as the region is covered by rapidly-alternating drought and thunderstorms. Go to Sootopolis City and speak to Steven to gain access to the Cave of Origin. Proceed to the lowest level of the cave and speak to Wallace to learn of Rayquaza's whereabouts.
Go to the Sky Tower just north of Route 130. Ascend to the top and awaken Rayquaza to stop Groudon and Kyogre from fighting and save the world. Continue through the game and beat the Elite Four and the Pokemon League Champion.
Go to the Weather Institute on Route 119 and speak to the head scientist who will tell you about either an extreme downpour or an extreme drought on a certain route. Go to the route and locate the new dive spot (when raining, it leads to Kyogre) or the new cave (when sunny, it leads to Groudon) and enter it to encounter the corresponding legendary Pokemon (level 70). Speak to the scientist in the Weather Institute again to trigger the appearance of the other Pokemon.
Using Pal Park
Deposit Groudon in the PC in "Ruby" or "Emerald." Save your game and then turn it off.
Put the "Ruby" or "Emerald" cartridge in the Game Boy Advance slot of the Nintendo DS. Insert the "Platinum" cartridge in the main slot of the DS. Turn on the Nintendo DS and load "Pokemon Platinum."
Select the "Migrate Pokemon" option from the main menu. Choose Groudon and up to five more Pokemon from your "Ruby" or "Emerald" game (unhatched Pokemon or Pokemon that possess HM moves cannot be migrated) and confirm your selection to begin the migrating process.
Open your game and enter the Pal Park facility. Go to the "Mountain" area on the northeast hill, run through the grass and you will eventually encounter Groudon. Throw the special Pal Park Pokeball to capture it. No battle is necessary since the Pal Park Pokeballs never fail. Capture any other Pokemon that you migrated and exit Pal Park, at which point you can withdraw Groudon from your PC.