Sylveon has the defenses to survive (more specifically Sp. Defense) but its main selling point is its hidden ability Pixilate, which turns Normal-type moves into Fairy-type and boosts their power. Most of these sets run Hyper Voice which is the most effective method of using Pixilate, although it does require transfer of an Anticipation Eevee (which evolves into Pixilate Sylveon) with the move from B2/W2 as it's a move tutor move in those games.