As for frustrating, Morty's Gengar. Miltank is difficult but Gengar would OHKO my entire team with Shadow Ball. Biff (my Pidgeotto) was the only one who could survive the Shadow ball being immune to it. So, setting up a strategy and inching my way through his HP, I finally defeated him.
I forgot about Morty's Gengar! That thing was a pain! Especially when it used Hypnosis + Dream Eater Combo! It'd take out my team like no tomorrow. Other than his Gengar the rest of his team proved to be particularly easy, although the support his Haunter gave his Gengar through Spite did prove to have a substantial impact when my most powerful move was dwindled down, and eventually wasted so I was left with the only option of chopping down his HP pixel by pixel.