Try a timer ball if you have any. Also, try to get it asleep/paralyzed, that will help. Don't get it burned/confused/poisoned, as it will likely kill it. If you want, there is always the Master Ball, if you are willing to waste it. Otherwise just keep on using Timer balls, Ultra Balls, etc
If you are having trouble, here is a video on catching Groudon: http://www.youtube.com/watch?v=sg3H62bRzo8
(P.S. Keep in mind that it is a legendary, therefore making it harder to catch.) HTH :D