Groudon uses Red Orb and stomps ovo.
But for real, Groudon probably has the skill advantage via being able to fight a legendary he's disadvantaged against for millions of years. Leviathan's water attacks aren't as good because of drought, and if he gets hit by fissure, it's all over. With moves like scary face and bulldoze lowering his speed, Groudon will quickly be able to shower him in attacks. His reduced speed allows him to spam Ancient power, which could boost all of his stats, use rest, which would let him heal up to full health while Leviathan moves slowly towards him and struggles to scratch him after all the debuffs. Beyond that, he has rock polish to increase his own speed. Thunder Wave to paralyze him, Slam him with a Solar Beam or Hammer Arm, etc. Moves like toxic also are annoying for the Leviathan since he'll be struggling to hurt Groudon with stat debuffs and chronic loss of strength.
Buut, despite all of this, a lot of the moves I mentioned take time to use/to have their full effect. If Leviathan just slams him with all of the Earth's Oceans at once then I doubt there's much Groudon can do. So it depends on what Leviathan starts off in character. Among that, he has things like Time manipulation too which will no doubt be handy, but I doubt it'll decide the match.
Basically, if Leviathan starts off with manipulating the Earth's Oceans. I'm voting for Leviathan. If he starts off with some small tier water attack or physical strike, then inconclusive, since it's anyones game from then on. Groudon could toxic and use his stat debuffs, but as long as it's still somewhat early in the fight and Leviathan has enough strength, Groudon could just get blasted with water from all sides.
That's my analysis. I'm not sure who to vote on lol.
