No real Pokemon players would use a Special Pokemon like Xerneas. Just to let you know. This team is totally beatable. Not even quite the charm. Anyone on the Seribii forums could take you out hands down. As for me I dont "hardcore" pokemon anymore but go on seribii.com and challenge players. I guarantee you're defeated :)
They're in the game, I don't see why legendaries are still considered to be cheap. It's not like every non-legendary is on equal footing - I doubt many competitive players would have caterpies and pachirisus on their team.