Actually, this is like Reshiram/Zekrom, you HAVE to catch the main legend. If you KO the legend, it will stay there and the game will say "It seems Xerneas\ Yveltal wants to help." until you catch him, which is fairly easy. That happened to me once with Yveltal, as I got Pokemon Y. So my source is past experience, which is the best resource in my opinion.