Gardevoir is gonna be your best bet in this battle. So be prepared to heal him constantly, maybe send in something bulkly(Muk) with Gardevoir and heal with that bukly pokemon and use Gardevoir for attacking. Of course setting up calm minds is kinda crucial. What could really help you is a ground pokemon, something with Earthquake. Now i used Golem but i really wouldn't recomend him since his speed is terrbile and before he's able to strike they'll knock him down, i barely beat her with that Golem. So a ground type with Earthquake or something and your Gardevoir, especially after like 2 calm mind buffs. Just keep him alive and wreck her team, only Drapion is immune to Psychic so be sure to knock him down with a ground type move