With Sylveon, if you keep spamming calm mind until it maxes out, while healing with ultra potions, you can easily tank its attacks since it only has fighting nd dark attacks. When you get put to sleep, just wait through it and heal when you get close to fainting. Once you've maxed out your special stats, then start hitting back with moonblasts. When I tried this the fight with this strategy I won first time (although you might have to get lucky on the first turn to get off the first calm mind) so good luck!