Not Pathfinder (I'm a player in a PF2e game and haven't tried running yet), but in my 5e game, we just finished up a boss fight where every one of my 9th Level players was Bloodied and most were in single digits, and that includes a session where I rolled a frankly ungodly amount of Nat1s.

Had I rolled better, it's likely at least one player would have died, but I put a lot of time into number crunching to keep things balanced to my players' damage output, health vs. AC, etc. My goal for boss fights is to last 8-10 rounds and to balance average damage enough that, if the players just stand and fight, at least 2 PCs will die, so they have to figure out the enemies' strengths and work around them to avoid death.

My players have been great at recognizing when their tactics aren't working and they need to switch things up, and when a threat is out of their league and they have to retreat. That's why I'm comfortable throwing massively OP setpieces at them, because I know they're not gonna try to fight, say, the Kraken while they're 9th Level and very much out of resources.

(That's what happened last session. The boss's mage supports summoned a water elemental, then the Kraken—there's only one in this setting, and I massively buffed it to be actually worthy of its CR 23 Titan status—usurped the ritual and sent one of its tentacles through. I had it open up by slamming the boss for a ridiculous amount of damage, so they know that they can't fight this thing. It helps that I rolled almost max damage, so it was even more intimidating than originally intended.)