Instructions
Visit Mr. Pokemon at his house, which is just north of Cherrygrove City on Route 30. He will give you the Red Orb, granting you access to Groudon in the Embedded Tower.
Fly to Cianwood City and head into the cave that leads to the Safari Zone. Exit the cave onto Route 47. You may battle a trainer with three Dunsparce on his team. Enter the Cliff Cave entrance next to the trainer, climb down the ladder on the right and exit the cave.
Select a Pokemon that knows the move Surf and surf west from Route 47. Continue heading west past the rocks until you reach the shore with a cave on top of a cliff.
Select a Pokemon that knows the move Rock Climb and climb up the cliffs. Enter the cave entrance to the Embedded Tower.
Proceed forward through the cave to find a Level 50 Groudon in the middle of the room. Save your game and press "A" to engage it in battle.
Battle Groudon and weaken it until its hit points are in the critical red zone. For a higher chance of catching Groudon, afflict it with status conditions such as Sleep or Paralyze.
Throw an Ultra Ball, a Dusk Ball or any other kind of Poke Ball at Groudon to catch it. If you are unsuccessful, continue throwing Poke Balls at Groudon until you capture it.