First, level up till 90 or more, then bring a Friend Bow and Oran Berries. When you reach the Kangaskan statue, go save your game, then continue. When you reach Groudon, hit him a few times, then throw one Oran Berry. After that, hit him again. Throw more Oran Berries, then take him down. If he wont join your team, turn off the game and restart!! :D