There's a bunch of mini bosses around that time too, but they're all pushovers. One of them, the seven swords, is a pair of samurai in a combined space, it's on the way to isshin and is actually avoidable. It can be infuriating but there's a simple trick where you take a stealth potion or whatever and mind control one of them.

Demon of hatred isn't exactly hard, even as he's super weak to a specific piece of gear, but it is long and because of that the opportunity for getting your foot stuck on a rock or hitting a wall or accidentally running off a cliff is amplified especially in that arena.

The Owl himself is actually quite easy because he has one attack that he over telegraph's an if you can play keep away and chip down that attack it's a breeze.

Isshin is just tedious and has the same terrain issues as DoH just not as bad. By the time I did it I came up with a sprinting route and geographical referencesto avoid getting cliffed. Still takes forever.