I found Lorelli hard because of her Lapras and Agatha because of her Gengar, but that's about it really. Lance's Kingra gives me a bit of trouble, but not that much really.
I think in R/B/Y when you used fire against things like Dewgong and Lapras it wasn't very effective, which annoyed me because it was part ice. That reminds me of when I first got Red and went against the E4, I used to have no idea what dragons weakness was so I had to do the rare candy cheat to level up my pokemon to lv100 and eventually won against the Dragonite by using Venusaur's Razor Leaf :laugh: