I'm gonna go with Diantha myself. Aurorus has some nice supportive abilities with the screens, as well as devastatingly powerful attacks like Blizzard and Thunder. Her team as a whole is packed with strong moves with strong Pokemon. Goodra with Fire Blast, Hawlucha with Swords Dance, Tyrantrum with Head Smash and of course, she has a Mega Pokemon.
Hawlucha can use a Swords Dance and get a massive power advantage over Cynthias team, while it may not one-shot her Pokemon, it would deal huge levels of damage to them, especially with attacks like Flying Press. Tyrantrum is also quite a powerhouse in his own regard. Her Gardevoir also has an amazing moveset on its own, but with the capability of Mega Evolution, it becomes all the more imposing.
Roserade, Garchomp, Gastrodon and Lucario are the only four Pokemon I can see really going for Cynthia. Roserade is strong, don't get me wrong, but it's also frail. Garchomp can be 1 shot with a good Moonblast or Blizzard and Lucario...well...nearly everything in Diantha's team counters him in some way. Gastrodon weirdly enough seems like Cynthia's "MVP" of sorts, it can tank most of what Diantha throws at it and still has good offensive options with Muddy Water, Earthquake and Stone Edge. But alas, Dianthas Gourgeist can take it out with Seed Bomb.
So yeah, Diantha with Mid-Difficulty. Can't really see her losing with her team composition.