Don't use Toxic and Leech Seed, if he doesn't use Rest he'll faint.
Edit:
Put your Kirlia in front, make sure it's fainted. Put a Pokémon with 157 HP and Final Gambit on your second spot. When you encounter Groudon, your sync gives you 50% chance of getting the right nature, but you don't have to switch to a different Pokémon. If Groudon faints after you did Final Gambit, it doesn't have 31 HP and you should Soft Reset. When you did get a Groudon with 31 HP, it survives Final Gambit and does a move, which fails. It'll give you a free switch. As next Pokémon, use Smeargle. Make sure Smeargle has these stats:
Sassy nature, 31|xx|31|xx|31|01 IV's, Spore, Rock Polish, Taunt, Recover, level 69
This Smeargle has 98 Speed at level 69. When you've switched in your Smeargle, use Spore. If you move first, Groudon doesn't have 31 Spe, since he has 99 with a Spe Neutral nature at level 45. If Groudon moves first, it'll either have a +Spe nature, or Neutral Spe with 31 IV. When it sleeps, use Rock Polish, and Taunt after that. If Groudon used Rest before you could Taunt, bring back your Final Gambit Pokémon to bring it back to 1 HP. If you did get Taunt up before he did Rest, the only thing you have to do is Spore it when it wakes up.