I hate it when n_d does this, especially when I first open up the thread...
Yeah, Groudon is good, but it is possible for Ky to beat him. Ray, however, doesn't stand much a chance against Groudon with Rock Slide, especially if Groudon also knows Swords Dance.
Anyway, back on Ky, it depends on which is slower. The fastest pokemon sets its ability in first, and since there can only be one weather effect at a time, the last one set in the battle would have effect.
If Groudon were slower, then it could Solarbeam away at Ky. But if, instead, that Ky was slower, it could use its amplified Water-attack (which for anyone with half a brain on Pokemon would be Surf, Hydro Pump, or Water Spout) and murder Groudon. But even if Groudon were still slower, Ky could use Ice Beam, and possibly freeze Groudon, leading Kyogre to victory.