Hmm...
Hardest: Until I worked out a pattern, the psychic leader Radomus, he just kept using Chess Moves on me and when I had a few fire and flying types, that sucked, he never once got Trick Room up though. Charlotte was hard until I found out Rain Dance ruins her effectively. I did have some issues with Noel, but TerraTwig just kinda made them go away. But then there was Shade... and his Chandelure... didn't have anything faster then it, so I had to resort to Noble Roar from Brave Heart to cripple it, although I do feel like I've done that a lot before... Serra was hard couse of 2 Pokemon, Cloyster and Lapras, everything else I was fine with except those two.
Easiest: Flobot, Luna or Samson, Torterra just destroyed them all with Curse/Seed/Eq/Razor Leaf, 1st shot all 3.(Please note I did get a 4x attack Razor Leaf crit the Cradily.)
So in order from hardest to easiest from the listed above:
Radomus, Shade, Serra, Charlotte, Noel, (skip a fair few leaders), Shelly, Aya, Flobot, Luna then Samson
*Edit* Aya wasn't that hard, more of the same, know what's gonna kill you then cripple it with BraveHeart, it works, well for me anyway.
Shelly was kinda meh, Emolga was clutch for me in that battle, having a Growlithe and a Pyroar helped a lot, just same, cripple then STAB kill after a few work ups.