Well as of now, that I'm a lot more experienced from playing it for 14+ years I don't have any problems really. But back when I had Crystal, which was around 12 years I always had a hard time with Whitney, also in Sapphire I had a problem with Tate & Liz, but that's about it. Norman was a little difficult cos of that Slacking but other than that he was easy. Well actually I had a hard time with a couple of the Kanto leaders in SS. I didn't have an electric Pokemon so Misty's Lapras swept my team... Crobat - Ice Beam, Dragonite - Ice Beam, Typhlosion - Surf, Lapras - Body Slam (mine didn't have Body Slam), Espeon - Body Slam/Surf... But I beat her when I stocked up on full restores lol. Surge was pretty difficult as well, and Brock was a little bit of a challenge.. His Kabutops somehow beat my Lapras and swept half my team. Good thing Dragonite had Aqua Tail.