As Gardevior and Sylveon share the Fairy typing, they both have a weakness to Poison, which no other Pokemon resists either. You don't have to centralize your team on these two Pokemon. I suggest you increase the variety of your typings, because it also opens up weaknesses to any Ghost, Ground, Rock or Flying-type move. I agree with PD that Greninja should replace Simipour on your team. That counts out one weakness. The only Pokemon than can deal with all the other weaknesses is Levitate Bronzong (Well, its not available in Kalos generally, it is via the Friend Safari, but I'm not sure if its against the rules if we were allowed to have this Pokemon).