That final bloody boss in the Brutal Doom Hell On Earth Starter Pack (20b)! I managed to 'beat' it, but only by saving my game 18 times in the process. I've seen a couple of YouTube clips in which players drift casually around the arena without attracting much attention from the numerous (or on these occasions not so numerous) monsters spawned by this boss, and who apparently never get caught by the huge rolling walls of fire he breathes out, but I'm more than a little sceptical about some of this. More representative perhaps was the Youtuber who awarded himself 10,000 health points via the in-game console, and still lost most of them before the end of the contest....
I recall also beating D'Sparil at the end of Heretic, playing on the highest difficulty level, and that it took about 20 minutes of continuous play.
Just to prove that I'm not a hopeless case as a gamer, I did complete the final level of Brutal Doom 64 on my first attempt, and somehow did it without losing a single health or armour point.