This game has this push pull effect when it comes to being aggressive. If you're not aggressive enough, enemies will just wear you down and break your posture or hurt you when you miss your blocks and dodges, because unlike Dark Souls they don't just stand there circle strafing you on a big cooldown, inviting you to attack. The gaps between their attacks and movement in Sekiro are comparatively short. (Or non-existent, in the case of the Guardian Ape.)
On the other hand, enemies that matter are designed to punish greed. That was my problem with Genichiro Phase 2. Get greedy, be out of position or mid animation when he does the lightning, get fragged. Also the same story with Shirafuji.
However the time I beat Genichiro, I just....R1'd him one strike at at time, no combos. I'd hit him, he'd jump back to do the bow, I'd hit him again, he'd go to swing, I'd hit him again, he'd jump back to use the bow...it was almost kind of laughable after all the running around and deflecting and whatnot I'd done prior. It almost felt like roll catching someone in Dark Souls.
So generally you just have to play very tight, be aggressive but also be ready to nope out or deflect or jump or w/e at a moment's notice.
Also it seems like what you have unlocked kind of matters in relation to some boss fights. I've unlocked about half the shinobi tree and nothing else.
On that note....I really like the economy in Sekiro in some ways. I've yet to unlock a ton of stuff because I die repeatedly. I'll get through a level section pretty well, but not have enough to spend XP on, then die a shit ton to a boss and lose it all. That's kind of kept my growth stunted, but it's also continually given me stuff to look forward to. Same story with the Tools Ammo. (I just call it Hearts.) I don't use my tools that often in regular play because I find the sword play a way more satisfying way to end fights. But then I try to get all I can out of them during boss fights. (The firecracker tool is a guaranteed spot of damage in pretty much every boss fight.) So I'll end up mowing through all my hearts. To be honest, the Firecrackers tool is the only one I use with any regularity. The rest are nice, and fun but often don't seem to be worth the effort to use.
That makes pretty much everything: money, XP and ammo feel meaningful because you can lose it easily. If you're the kind of person that is motivated by unlocks, and likes your money to have context instead of just "a big fat pile of gold always sitting around" Sekiro really makes the scarcity feel meaningful....unless you're super good I suppose and rarely die.