I found Lenora the most difficult. STAB Takedown? OUCH.
I had a Timburr and I still had difficulty.
Clay was a bit underwhelming. The flying gym was a let down (seriously, not a good type for a gym).
Iris would have been beastly had it not been for my Fraxure dragon dancing her to oblivion. I reckon I would have had to have trained Cryogenal or Beartic to take her down otherwise.