Listen up all you faggots! Have you already sold your soul to our new glorious overlord Xerneas? This faggot can do anything and I'm here to fucking prove it. Let's start with the most obvious one:

Herbneas:
>Xerneas @ Power Herb
>Geomancy
>Moonblast
>Pick two: Thunderbolt / Focus Blast / Psyshock

Switch into some faggot's Outrage, use your free turn to double your SpAttack, SpDefense and Speed and start sweeping. If there are pebbles on your opponents side, Xerneas doesn't have to give a shit about Focus Sash, Sturdy or Multiscale. Thunderbolt hits faggots like Ho-oh, Focus Blast punishes Steel types and Psyshock demolishes Blissey and Chansey. After just one turn of setting up, Xerneas outspeeds even most Choice Scarf users and you're in some goddamn trouble if your Extremekiller Arceus can't OHKO this beast.

While most people think Xerneas is made to be a special attacker, it has nice 131 / 131 mixed attacking stats and an impressive physical movepool. Consider the following:

Punchneas:
>Xerneas @ Life Orb
>Outrage
>Close Combat
>Horn Leech
>Whatever you want

Since Dragon and Fighting already offer perfect coverage against anything else besides Fairy, the last two moves serve very little purpose besides countering opposing Xerneas. Horn Leech allows Xerneas to punish the omnipresent Kyogre by hitting its crappier defense with a super effective move while healing itself, and it can be used to steal shitloads of health from Blissey too. For a more unorthodox set, one could combine this set with Geomancy and Power Herb since doubling your special bulk and Speed is never a bad thing.

Or why not go all out mixed for fucks sake?

Mixneas:
>Xerneas @ Life Orb
>Moonblast
>Thunderbolt
>Horn Leech
>Close Combat