I found all of them pretty easy (especially Brycen. Seriously, in Black I swept him with an underleveled Stunfisk). However, I can see how in BW1, Elesa can be fairly annoying if you don't prepare for her two Emolga. However, if you have a counter for her Emolga it can be a fairly easy battle. I liked Cheren in B2W2 because I was only using my starter, so it was more difficult (as there were no Pokémon early on that I wanted to use). I could however, have trolled him with a Drifloon I got from Dream Radar if I wanted to though (I was nice and didn't xD).
I can vaguely recall Roxie being a pain, because of her Whirlipede. I also didn't use Lucario or Magnemite (as I used them in earlier games), so that made it harder.