Well, three things worked from me. First, the Sylveon that Ren gives you at Alamissa Urben can setup Misty Terrain, boosting it's attacks, and preventing sleep. Second, the Chesto Berries sold at Hiyoshi City can trivialize the support of Ralts and Kirlia. Lastly, a Magical Seed can automatically set up Trick Room, which helps a lot if your team isn't outspeeding Gardevoir (which is always the case, haha)