>>43697761I was a bit overleveled for most of the story so I didn't have many issues with boss fights, although Evice gave me some trouble because his Tyranitar was a massive pain to catch.
The hardest fight was actually pic related, some random trainer in one of the Battle Mode colosseums. It took me 4 or 5 attempts to beat him because he'd always lead with a Perish Song user, trap my lead with Wobbuffet so they fainted from Perish Song, then have his Breloom put my Pokémon to sleep and spam Double Team so I couldn't hit him back while he punched my team to death. Even if I immediately switched out of his Perish Song user, they'd either dodge my next attack or tank it and then use Perish Song again. I ended up winning on an attempt where he selected Shuckle (who wasn't as much of an issue as everyone else) and I had a little more luck with RNG.