Magical seed is great, especially with Shiinotic and Sleep Powder. Ralts and Kirlia (Fairy/Poison and Fairy/Fire) are both weak to ground so bring a pokemon with Earthquake if you can, Shiinotic even resists it. Gardevior will attack last under Trick Room, should fall after a couple super-effective attacks (weak to Steel, Poison and Fairy).
The Ralts and Kirlia all have the ability Friend Guard, reducing damage dealt to their allies. If you can neutralize their abilities or have a pokemon with Mold Breaker, both kinda difficult to do, the fight would be easier.