I'm really disapointed with the bosses so far in Nioh tbh, the main reason being what you touch on here - everything is so reactive, and it's hard to be pro-active.
The levels themselves are fucking brilliant. Absolute top notch design, balance, challenge, the lot. Same with standard combat, superbly designed and a ton of fun.
But the first two bosses are utter bullshit. It's just SO fucking boring having to chip away at them for so long on based on reactive situations. Not only can't you pile in for any heavy damage, but you have to wait for them to make their moves first before you do so. There's no risk-reward situations, there's just "it's gonna take you ages whatever happens, and if they decide to one-shot you in the meantime that's life"
Poor boss design IMO. Just makes things dull. I was having a blast with the game itself, but won't continue now. It feels more like work than fun.
That's because you suck. No, not you as a player, your character sucks. The 1st run is just an extended tutorial - and it's the worst part - the real game starts once divine gear begins to drop and you can put together a set that complements your fighting skills. Throw in some magic and ninjutsu in your build, proper gear with the proper bonuses, a good Guardian Spirit, and the way battles go will change. A lot.
This is one of the beautiful things about Nioh, and one reason why it's combat is miles ahead of Souls games. No matter what bullshit the bosses throw at you, at some point you gain an equal, if not greater, amount of bullshit in your arsenal.