Clannfear, their reflect damage ability is dangerous because i always play on highest difficulty. Much more dangerous when summoned by Xivilai, and to make matter worse, i gotta deal with 3 or 4 of those deadly combo.
Wiil-O-The-Wisps, for sure. Them going invisible is annoying as hell, and having to go to a shrine (or be lucky and have the right potion on you) each time they damage an attribute is a pain.