Naughty nature isn't good for Xerneas, tbh, because its main STAB, Moonblast is a special move. It doesn't learn Play Rough either, though it does get some physical coverage moves. I'd use 252 Atk/4 SAtk/252 Spe.
Thanks! I wish I would've paid more attention to its nature before I continued the game, but I wasn't even thinking about natures at the time. I most likely won't use him that much, but I'd like to make it somewhat decent. I haven't put much effort into EVs because I was planning on transferring all of my trained Pokemon from Black 2, but Poke Bank had other plans. xD