Getting Over It with Bennett Foddy is an interesting moment when it comes to level design in hard games. By "Hard Games" I mean unforgiving games like Dark Souls or Super Meat Boy, where failure and challenge are aestheticized as a core part of the experience. Getting Over It distills the crushing setbacks in the Dark Souls failure loop into a "you against the mountain" (which is really "you against you") struggle.
Interestingly, I don't find Getting Over It to be particularly frustrating. This is coming from someone who, in an embarrassingly recent memory, gnawed on a controller during a particularly arduous Shadow of the Colossus battle. Every mistake in Getting Over It is your own making. You're not at the mercy of pernicious AI or Dark Souls' capricious invisible hitboxes; if you fall, it's because you missed a jump or pushed yourself off.
It's kind of odd to talk about level design in this game, since it's presented as a continuous mountain, but it's divided up into discrete challenges - each one with a particular failure possibilities. Biff a swing in the Chimney, for instance, and you're set back maybe a few minutes. Falling off the wall of lawn crap can send you down a chute that can take hours to re-traverse. Obstacles are organized around a visual theme - a rock face, a construction site, an icy summit.
"Orange Hell" is one such segment - the closest Getting Over It comes to a boss battle. It's completely psychological; the obstacle itself isn't particularly challenging, but the cost of failure is high. It's like walking across wide plank of wood - no problem when it's 6 inches off the ground, but a nail biting experience when you're 30 stories up.