Ugh, Isshin the Glock Saint. Thorn in my side, as someone who loves the entire soulsborne series, beat most of them (not including DLCs) - he did break me. Sekiro is one of the tightest, most well designed games, it's super fluid, you feel incredible and animations are on point. This game is the hardest of the series because it is not something you can stat-pad against at least not as harshly as you could grind on the other games.
I spent many, many tries against him to get to the later phases but still would die. Like you multiple days, many sessions but against Isshin.
I moved on and came back to my save years later, that muscle memory was lost and did way worse. Realizing this may be a boss I simply don't conquer.
Elden Ring has sparked joy.