Right now, I'm on the Crystal Guardian. Comparatively, the boss itself is not going to be that hard; I think I've got it down. But it takes one or two tries to learn the pattern for most bosses, and then a few tries to execute on that learning, at least for me. This boss occupies a bench that I can't use until it's defeated, so I have to navigate a long distance upward all while contending with traps full of those laser crystal caterpillar things, where one hit knocks you back into two other lasers. Getting to the boss with full health is a time-consuming challenge -- it's a slog. It took a while for me to get back to the boss directly before this (The Mantis Lords), too; though the trek wasn't as challenging, I had to try that boss a lot before I was successful, which means I had to make that trek over and over and over. And as Skiptastic said, Soul Master is another example. I do think it's a pattern.
Yeah, this is where I'm at currently (including the part where I still love the game). For me, game design that forces you to repeat a long series of tasks that you've already successfully completed a dozen times just for the chance to make an attempt at the one task you're stuck on is simply not good design. As a gamer who has not enjoyed that sort of pattern since the NES days, it now strikes me as a holdover from the days when save systems weren't as advanced or when games were still in the proximity of that quarter-munching mentality, seeking to cheaply extend their length and challenge. It's just a joyless process to me.
But I brought it up here because I know people have really studied Hollow Knight deeply, so I was curious if there might be some sort of design justification for it in this case. I'd really like to know if there are people who think it adds something to the game (and, no, "sense of achievement" doesn't cut it for me; I get that from the boss itself, not walking there for the 14th time), or if the general consensus is that it's an antiquated choice and a mark against an otherwise wonderful game.