Crossy, Mystic Water sucks, those like 10% increaser items barley change a thing, your better off with berries. Hmm Groudon don't really need Bulk-Up they already have high defense base stats, but if you have a nature that does not benefit it I can see. Point in case is that Sword Dance would be better going against a Kyogre, even with the sun out Groudon most likely can't take 2 hits with Water attacks. Here would be the format IF Groudon was faster.
Turn 1:
The Groudon used Sword Dances- Attack sharply rose.
The sun is bright(whatever I am tring to reenact it)
Turn 2:
Kyogre used Calm Mind or Surf/Water Spout/Hydro Pump.
The sun is bright.
Turn 3:
Groudon used Earthquake. Kyogre faints.
The sun is bright.
We know Groudon can't beat Kyogre without the sunny weather, that is why you sacrifice a useless Pokemon and then call out Groudon, if Groudon is fast enough to override Kyogre's Drizzle ability.
Rayquaza is the weakest out of the three I think in my opinion. Kyogre could destroy it with Ice Beam, and Groudon could whip it out with a Ancientpower/Rock Slide. There is a slim chance that Rayquaza could whip out Groudon, but not very likely though.
Yawn - what is there to say?