Instructions
Buy at least 30 Ultra Balls and Timer Balls, as well as healing items such as Revives and Full Restores. Move a level 50 or higher Pokemon that knows a sleep-inducing attack such as "Spore" or "Sleep Powder" to the front of your party.
Walk up to Groudon and save the game. Engage Groudon in battle and put it to sleep. Whittle down its HP into the red zone, and put it to sleep again if it wakes up.
Heal your Pokemon if they are low on health or have fainted. Throw Ultra Balls and Timer Balls at Groudon until it is caught. If you run out of capture balls or healing items, reset the game and repeat steps 2 and 3.