This time yesterday I would have had a different answer, but last night I was running through with a POW (plain old warrior) and did Treva's Watch (the one where the guy outside hires you to go clear it). This dungeon, for some reason, has two bandit chiefs, and the second one has to have been the toughest single opponent I've ever had, levels notwithstanding.
The chief was wearing steel plate, and this was at a level where the best armor my char could get without grinding was regular steel. I had all my skill levels in heavy armor, block, and one-hand, which were all around 40. For perks I had shield bash and one handed perks up to and including the mace perk that ignores armor.
Anyway, this frickin' boss was immune to my damage. Literally, mathematically immune. I ran at her, bash, bash, power bash, power attack, and her health bar didn't go down so much as a sliver. I used a mass paralysis scroll, which froze her for ten seconds, during which time I literally whaled on her non-stop, and her health bar did not go down more than a hair's breadth. Meanwhile, she was wielding an elven dagger and was one-shot killing me. This meant that I somehow had to find a way to kill her without taking a single hit. I assume she was glitched and spawned at a level many many times my own.
I actually managed it, finally, by luring her down the hall and over two bear traps. Bear traps appear to do a set amount of damage regardless of armor. It's not much though. I had to lure her over them, bash her to stagger her, reset the traps, and lure her across the other way, again all without ever getting hit once. I had to get her to hit both traps about twenty times before she finally died, and it took probably close to 200 reloads before I finally got it right.
All in all, it was not nearly as satisfying a triumph as I thought it would be, and if I had to do it again, I would just set the difficulty down to novice. I haven't had to do that yet though, and pride, I guess, kept me from it then. Stupid.