Sadly, Pixilate is going to be completely useless on Sylveon until Pokebank is opened up and Eevee can be transferred with Hyper Voice, so go with Cute Charm. Also, the Calm nature on the first set looks a bit misplaced seeing as how you're trying to invest as much in physical bulk as possible. I recommend you change that to Bold.MegaGardevoir: 124 Attack, 177 Sp. Attack
Aegislash: 171 Defense, 170 Sp. Defense, 167 HP
Moonblast (95 Fairy Special) prediction: 28-33 damage.
Damage: 31, 29.
Pixilate does nothing to moves that already are Fairy-type.