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Cynthia as Champion is the only actual hard battle in the entire series. You know why? Because the AI is actually GOOD. It takes god decisions. First time I battle her I would reload my save and change order of Pokémon in my team or something like that and she would react accordingly.
Other fights are just illusion-hard. The totems can fuck you up simply because it's two against one. Ultra Necrozma is high leveled with high stats for that point of levels in your team. Gym leaders? They'll be hard or not depending on your team, I've had the same battles gone great or gone horrible.