Notice that mega gardy dislikes repeated psychocks specially with hazards on. She is barely a check hoping to come on a Dmeteor.Bulky Fairy types like Chefable and Sylveon will be great partners for Volcanion on Stall since they can easily take attacks from the Latis and hit them back hard with Moonblast and Hyper Voice, respectively.
Mega Gardevoir will also be a good teammate on more offensive teams since both STABs from the Latis are either immune or resisted.