Bring seeds to put him to sleep. Then also bring a pokemon with rain dance to raise the damage of water type moves. Try to avoid pokemon that are weak to his moves. The extra boost from the rain and the fact that you keep putting him to sleep should make it an easy battle
Are you fighting during game play or after game play with the wonder mail codes? If you are fighting groudon during game play make sure your partner has a strong hitting move and you have a strong move or a long range item you can throw.