Shamen, if for no other reason than they can turn God Damn Bats into a Demonic Spider by enchanting their weapons, and can ressurect those same enchanted enemies.
Game Breaker: The High Elf Clerics are A:) Melee Powerhouses, B:) Able to easily heal themselves and allies, and C:) break the "normal" balance by being more Combat Medic than Squishy Wizard or Glass Cannon. The only thing that keeps them in REMOTE balance is the fact they can't wield bows, and have weaker ranged spells.
That One Boss: Pelys is notoriously more difficult to kill than any other boss, INCLUDING Inoruuk especially if you're playing a low level Squishy Wizard. And he's the first boss you fight!
Mithaniel Marr in Return to Arms. He has an attack where he leaps at you with his sword, which does heavy damage, tracks your movement, and knocks you to the floor. If he lands this even once you're pretty much screwed as he will proceed to spam the move before you can get up until you die.
He is only that tough due to copious amounts of Fake Difficulty, 90% of his attacks can break your blocks and knock you down. Oh, before you get any ideas of just running from him and shooting him, he will catch up, and proceed with the above.
Every freakin' boss is MUCH tougher than any of the enemies. More so if you have a "melee-only" Glass Cannon class.
That One Level: The absolute worst level in Return to Arms the is the Plane of Water on the Good alignment path. It requires you to push three blocks all the way to a series of holes in three separate, long caves, with your movement speed reduced as you're underwater. To top off how boringly tedious it is, there's a bug that affects both the Good and Evil versions of this level that can prevent the necessary quest item from appearing (if the shell that's supposed to drop it happens to contain a bomb), forcing you to redo the entire level over again.
I beg to differ, the worst level in the entire game is the Plane of Storms, there is a part where you have to sneak past Invincible Minor Minions that double as the Demonic Spider of the level, as they can and will One-Hit-Kill if they spot you, and after you get the shard, you have to go back and fight them! The only good thing about this level is the fact the golem enemies here deal much less damage than their bretheren in the Plane Of Fire.