Personally I find Xerneas quite underwhelming so far. It is a sweeping that doesn't bring much to the table in terms of synergy or revenge killing capabilities. With Defog everywhere, mons like Whirlwind Ho-oh and Lugia make Xerneas useless should it set up. Thunder Wave weakness is also unfortunate. Truth be told you should be able to fit in any of the following: sdef Kyogre, Ho-oh, Lugia, support Arceus with roar or toxic, Amoongus, Jirachi, Bronzong just to name a few on your team. Personally I find it that if you are well prepared against it, it shouldn't be hard to handle, but of course, just like many end game sweepers, Xerneas used by a good player might sweep another good player once in a while.
Anyway, enough about Geomancy Xerneas. I have personally been trying scarf Xerneas, which is terms of power is a little bit underwhelming. The amount of threats it can revenge kill is, however, quite a big deal. Just a quick list: Darkrai (can even come in on Dark Void if you are running Sleep Talk), Darkceus, Fightceus, Mewtwo (all formes if you are running Megahorn), Zekrom, Palkia, Kyurem formes, Reshiram, Rayquaza. It does face difficulties with low initial power, though Fairy Aura boosts the Moonblast by a bit at the very least. I have been tinkering around and tried the following set:
Xerneas @ Choice Scarf
Ability: Fairy Aura
EVs: 224 Spd / 76 Atk / 208 SAtk
Naive Nature
- Moonblast
- Megahorn
- Rock Slide
- Sleep Talk
Though there are many options like hidden powers, Close Combats, Tbolt etc.