So the rift gardevoir, alongside Angie are the two banes of my existence. Gardevoir is stupid fast, hits stupid hard, has a stupid ability and fights in a stupid OP field. I actually dislike this fight because there's no reason for all the unnecessary buffs to it.
Nevertheless, if you go in without a strategy, she will most likely one-shot all your mons. She carries a Lum Berry so she'll avoid an initial status effect. Perish song doesn't work because of her 'aura'. For a lot of mons with Sturdy she'll use lovely kiss to put you to sleep and chip off a bit of HP [terrain effect] and THEN use her OP move to one-shot.
So what's the solution. Luckily the game hands you both the solutions - Mom's Sylveon and the Chesto Berry. The former is given to you in the desert, and the later is available in the shopping section of Hiyoshi city. There is an audino trainer in that section as well. This strategy assumes you have some extra cash on hand to work with because you'll need a fair amount of Hyper and Ultra Potions. I like to have 25 plus Hyper potions and around 10 ultra potions on hand. Train your Sylveon up to level 80 with the audino trainer, make sure it has a damage dealing fairy move, either moonblast or pixilate hyper voice is fine. Having calm mind is also very important. Luckily she comes with calm mind so as long as you didn't remove that, you're good.
Equip the chesto berry on sylveon and start the fight. Gardevoir will go for lovely kiss [LK] and your chesto berry should get rid of the sleep. Use calm mind. She'll use LK again, its fine. Just wait out a few turns, constantly healing to keep your HP full. My non-EV trained Sylveon could tank 2 hits from Gardevoir at least, heal after every hit though and on alternate turns where your HP is full, go for either moonblast or one more calm mind for extra insurance. After 2 calm minds, heal up again, and once you break free from the sleep, go for those moonblasts. You should take out one shield with each attack and after that its just moonblast/Hyper voice central. Make sure you're constantly healing though, if Sylveon dies, the strategy is ruined.
Rinse and repeat till she's dead on the ground. It's a cheese strategy but I personally find how OP this mon is to be pretty cheese too. So I don't feel bad whatsoever.