I thought I was ready to take on Dagoth Ur at level 18 with a few dozen miscellaneous potions and enchantments. I got hit three times and died, even with every single enchanted item in my inventory active.
I still eventually beat Dagoth Ur, on somewhere around the fifth try. Notable moments include having to duck back in the other room and sleep for 40 hours to recover my HP mid-battle.
The Elder Scrolls III: Morrowind.
"Hey, bud? I'm getting a little tired. Mind if I step outside and sleep for a couple of days?"
"Sure thing, I've got nothing better to do. Let me know when you're back, and we can finish this fight."
...actually, I'm embarrassed to admit, this is how my much-younger self beat the final boss of Neverwinter Night's first chapter. My companion had died, I'd suffered lots of stat penalties from disease, I hadn't saved recently, and I didn't know if there was a route back to town to revive my buddy and cure stat penalties. So, I'd open the door to the boss room, hit the boss a couple of times, then hit the boss with Knockdown (disabling him for one round), shut the door, exit combat, and enter stealth mode. With no enemies in LOS, stealth succeeded automatically with no check, so the boss 'lost sight' of me and I was free to backtrack a bit and rest. Rinse/repeat for a half hour, before the boss finally keeled over. I never moved more than 3 steps into the room.