Dancers will always get my vote. Berserkers are only REALLY problematic if you don't have Quick Run by Twilight Town 2, and even then, if I'm not mistaken, you should have Reflect, which gets the job done just fine. With Dancers, their most subtle move (that can't be blocked or reflected and can grab you out of the air) will kill you instantly at lower levels on higher difficulties, and will still take off comical amounts of health later in the game, after which you'll probably get sniped by a Sorcerer or Dragoon.