Go with Zacian
In-Game, there isn’t much a point in setting up when you can just KO the Pokémon anyways. Zacian has amazing attack and Intrepid Sword, so the majority of in-game opponents won’t live a hit from it. Xerneas can’t hit nearly as hard until it uses Geomancy, and as I mentioned, setting up is a waste of a turn when you can OHKO most things. As for the moveset:
Zacian @ Rusted Sword / Choice Band
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Ability: Intrepid Sword
- Play Rough
- Iron Head / Behemoth Blade
- Close Combat
- Wild Charge
Full coverage.