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Couldn't find this anywhere in the search and not sure where to give this information. I verified that defeating Xerneas in the first battle and then saving does not lock it's stats/nature. This lets you more quickly reset to try and get an ideal nature and IV spread. Should work the same with Y.
Thank you. In an internet full of trolls, hopefully I can trust a mod here. ;) That cutscene is killing me. I'm going for perfect 5iv (modest) no excuses...already dumped one that was a mere 1 point off in 1 stat.
What nature did you go for?
Edit: Just a little update but yesterday I bagged a 31/xx/31/30/31/30 Xerneas. Modest and HP fire. I was going for all 31s except for attack but this is so much better imo