Oh where to start!
Let's see, Shade first of all, his ghost type pokemon mixed with electric moves with the field type he has, kinda fun.
Noel, never thought I'd have issues with a Normal-type leader, but Skill Link Chinccino too op (had to catch a Throh in Pyrous Mountain to beat it)
And lastly, Charlotte, simply because of one fact: I never raised a water type Pokemon, which means I had to go at that Gym with Charizard, which needless to say was useless, Gardevoir which was still one of my best bets, Ampharos, Golem, Absol and Beartic.
Beating her was the worst, my strategy was made up of Reviving, Trick Room and Blizzard to get rid of the field effect and then use Gardevoir/Ampharos to deal with the pokemons themselves, which took a few attempts.