Candice was uber difficult for me. Her Mamoswine knowing earthquake and me having a Houndoom. I should have gotten a fighting type, especially after realizing most of my Pokemon were weak against ice and ground (Staraptor, Torterra, Luxray, Drifblim, Houndoom). Surprisingly, my Gastrodon one me that battle. I don't even remember how. A lot of lucky shots, I guess.
Fantina was pretty difficult, but my Torterra and Luxray knew crunch, which blew through her party. The others were 'meh'. Roark, of course, was easy. Gardenia didn't provide much of a threat. I only had to fight Wake once. Byron was easy, as has been mentioned. And I never had troble with Volkner. That fighter chick was easy, too, but her Lucario gave me a bit of a run. What the heck is her name? Wow, I'm drawing a complete blank...