The almighty Deer God, Xerneas has been a terror since its introduction thanks to the sheer potency of Geomancy. 2 Quiver Dances at once would turn many Pokémon unmanageable, let alone one as excellent as Xerneas. Base 131 Special Attack with the excellent Fairy STAB getting another boost is threatening even both the Geomancy boost, and +22 base 99 Speed outruns the entire unboosted metagame except Regieleki with little investment. 126/95/98 defenses are already above average before the +2 to Special Defense. All these factors means that Xerneas can decide entire battles after a Single Geomancy.
However, Geomancy has severe restrictions in its use. You are more or less forced to run Power Herb, costing an item slot. Additionally due to the vanilla two turn charge, if the Power Herb gets Knocked Off, Xerneas gets forced out or Hazed, then Xerneas does not have much left -it lacks the reliable recovery to get a second shot at a Geomancy and its power and speed while good fall short of many faster threats.
Them's Fighting Herds!
Xerneas
Ability: Fairy Aura
Item Attached: Power Herb
-Geomancy
-Moonblast/Dazzling Gleam
-Protect
-Substitute
EVs: 4 HP / 252 SATK / 252 SPE
Nature: Modest (+SATK, -ATK)
What's that? Xerneas only has one strategy in doubles? Them's Fighting Herds! Xerneas' game plan is in theory simple in doubles. Use Geomancy, sweep with Fairy STAB, Moonblast for more power, Dazzling Gleam for Spread. However, it is far from that simple. Steel types are everywhere in VGC, especially Zacian Crowned which eats alive even Dynamaxed Xerneas. So Xerneas requires intense support in order for its win condition to go through. Substitute helps with that, guarding against predicted switches into Zacian or other Steel types, protecting against predicted Spores and Sleep Powders that can ruin a sweep and when combined with Protect, keeps Xerneas alive long enough for it to exercise its win condition.
Due to the heavy Anti Steel support needed, Xerneas fits naturally on Sun teams. Not only is Groudon a fantastic partner due to the Fairy-Ground combo, it enables dangerous Sun sweepers that Zacian and other Steel types want nothing to do with lest they get melted or grounded. Xerneas in turn is a potent win condition that doesn't need the sun up to function and matches well against many Dragon types that resist the Fire/Grass offense Sun teams employ.
Other Options
Hyper Beam -Dynamax for a massive Max Strike that provides additional speed control.
Thunder -Kyogre ruining your Sun friends weather? Show Kyogre that is a grave mistake.