I honestly only had a problem with Whitney the very first time I battled her. After that, I learned quickly that Geodude and Machop both decimate her team, especially with a lucky Magnitude 8-10. Resistance to Rollout (and Stomp in Geodude's case) is a good thing. Even if it does manage to take out Geodude or Machop, it's usually weakened enough by that point that Kadabra can revenge kill it with Confusion or an elemental punch.
For some reason I recall having trouble with Fantina a few times, though I can't specifically place why. Either way, only Fantina and Whitney ever posed a challenge, and I've lost to Fantina more than to Whitney, so my vote goes to her.