Facing Norman for the first few times is so annoying due to his damn Slakings+use of hyper potions :/
The 7th gym leaders were also annoying for similar reasons.
I feel bad forgot the name. But the electric type gym leader. Even with aron I couldn't beat him. I also had trouble with twins a while ago but did again first try won but yeah that's about it.
Wattson and Lize & Tate seems to be the concensus, the difference was, I was at a good level for Liza and Tate, not so much for Wattson.