Game Breaker: though they don't exactly break the game, fire-enchanted weapons are probably the most effective of all weapons by the massive damage they deal.
That One Boss: Ironically enough, the first boss could be considered as this, mainly because of his meteor storm. Bosses two and three can be hard to figure out, but they're almost nonthreatening once you do know the tricks to beating them. The Gnome King is a tough customer, but with a good weapon it all comes down to just managing your rage and resources. With a little bad luck, however, the Mage can make your life miserable even if it's not your first time playing. For an even greater challenge, try to beat him without raging and see how that goes for you.
That One Attack: The reason why the Mage can be a pain to deal with. Even though the spots where the fireballs will land are marked by shadows, but they are not at all representative of the deceptively large area each fireball hits. The shadows themselves can sometimes be hard to spot until it's too late, and you don't get that much time for running.