That dungeon where you fight
Spoiler Dragon Priest from the mages guild questline
I couldn't beat him with my melee fighter without cheating, he literally insta-killed me with his lightning over and over again. :S
The second time I fought him, I was a Breton mage (25% magic resistance) with the atronarch sign (50% magic absorb). Now THAT was an easy fight. You might even say it was a relaxing experience...

do I need to tell you how much trouble a destruction mage has in the world of Skyrim.
Weird, everyone keeps saying that, but I'm currently playing a destruction/summoner mage and I find the game incredibly easy. I'm level 23 now and I'm thinking about creating a new character, because the game is just TOO easy... I play on the adept difficulty.