Prankster

In a Prankster user you want solid bulk and a type that isn't weak to very many things, and Xerneas fills both those fields. Moonblast is also very helpful because it beats Dark-types, allowing Xerneas to viably run moves like Defog that would be terrible on other Prankster users. Alternatively, even this very defensive Xerneas set can form a thematic hazard-stacking core alongside mons like Primal Groudon or Poison Heal Regigigas, with the idea that no Magic Bounce user can account for both at once. Offensive items like Fairium Z and Pixie Plate take this concept further, punishing Fairy-neutral or Fairy-weak defensive mons that are attempting to Recover stall Xerneas. Shed Shell helps Xerneas escape mons like defensive trapper Primal Kyogre, which could otherwise suddenly remove it.

Haze is the standard on Prankster to cover Dazzling and Dark-type threats, but in a modern, post-Shell Smash and Belly Drum metagame, Topsy-Turvy and even Encore can be considered. Very aggressive teams can even try forgoing recovery altogether, though this is very speculative and not enough modern games have been played to test the mettle of this idea.

-trollkastel seal of approval

Sets like this are usually tossed on a team in order to not auto-lose to e.g. Simple setup, but this is still a Xerneas, it can still click STAB Moonblast at the enemy and sometimes that will force them into an exploitable, defensive position. This Xerneas is best used against very setup-reliant offense teams to buy time and not get run over immediately. Against more defensive teams, use Xerneas to fulfill specific positional objectives, like removing hazards with Defog, as needed. Note that Prankster helps Xerneas stay relevant even at very low HP.

Dazzling Mega Gengar kills you, although I don't know if anyone still uses that. Ignoring obvious Xerneas counters (Steels, Primal Groudon, Gengar in general if you lack Glare), something annoying for this set specifically is Triage. Beating setup sweepers is Xerneas's job, but it takes massive damage from +3 Oblivion Wing. The Calm nature is meant to mitigate this as much as possible, taking Mega Charizard Y's +3 Oblivion Wing from 68.8% to 18.8% to OHKO from full, and Yveltal's +3 Oblivion Wing to a near-guaranteed survival after Rocks (assuming Life Orb on both), but this still leaves Xerneas pressured to stay exceptionally healthy. Triage is broken dude you gotta run like Kyogre or Nihilego or something