I kinda used hax...
Caitlin, I don't know, I used Zoroark disguised as a Conkeldurr. XD She kept using Psychic but it had no effect. I've heard her Musharna is troubling.
Shauntal, another sweep with Zoroark. Chandelure is the one to watch out for here. And her first Pokemon, Cofagrigus isn't tough, but annoying. XD
Grimsley, easily taken down with any fighting type.
I think the one that gave me the most trouble was Marshal. His Pokemon always survived with 1 HP, and when he gets his punches and everything on you, it's hard.
Btw, I had level 50s.