EE:The Forgotten One
The Forgotten One is a rather deadly boss despite only being "normal" difficulty. As soon as he takes damage, he activates "Soul Sword," which bumps his attack to 150, gives him Retaliate: Fireball, and makes all monsters on the map No Experience. Killing a monster will remove Retaliate: Fireball and lower attack proportional to the monster's level (bottoming out at 1), and attacking The Forgotten One will restore Retaliate: Fireball and increase his attack a bit back toward 150. The key is to leave enough monsters to defang The Forgotten One long enough to cleave through his rather sizable health.