A boss you died to 1000 times, an enemy that still strikes fear into your heart every time you replay your favorite game, etc.
For me it's the Blood Starved Beast from Bloodborne. I died to it probably over 100 times my first playthrough no matter how many times I replay the game I still get torn to shreds repeatedly. Honorable mention cazadors from New Vegas
I've only ever soloed Midir using the pestilent mist strategy.
Basically, cast mist and then stay right under him but out of the mist so he stays inside the AOE. If he flies off, chase after him before he does the laser breath thing.
Since mist takes a fixed percentage of health from its target each tick they stay in the cloud you can just cast and focus on dodging the attacks as Midir's health chips away.