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Assuming pokemon can change movesets, natures, EVs, etc. for each opponent.
Gallade can't OHKO charizard X but zard X can OHKO gallade in return. Gallade would need a stone edge crit to win.
Mega lucario can't OHKO mega metagross without a crit, but mega metagross can 1HKO in return.
Mega metagross cannot OHKO mega charizard x unless it gets a crit or uses rock slide and the charizard doesn't mega evolve but there's no reason for it to do that so charizard x moves onto finals.
Charizard y vs mega gardevoir is the most important fight in the tournament and it comes down to gardevoir hitting a thunder. If it wins the speed tie and lands it (in the sun, mind you), it moves on. Otherwise charizard y can OHKO it with overheat or blast burn.
Mega blastoise OHKOs even max invested mega garchomp with ice beam >80% of the time and it can't do much about that without a critical hit outrage, and even that's a roll.
Now, if charizard y beats gardevoir, mega blastoise wins this whole thing, because it can live a non crit solar beam and with full investment it can rock slide mega charizard y for a 75% chance to OHKO and follow with an aqua jet, then dragon pulse mega charizard x for the win. It could also just mirror coat a solar beam. But if garde wins against mega charizard y, it should win against mega blastoise, and mega charizard x vs. mega gardevoir ends with a flare blitz OHKO.
So either mega charizard X or mega blastoise wins. Really thought it'd be metagross from first glance.