>>16051359
Sounds like Muneshige. He's well-known for breaking many of the established rules for human bosses. As he can cast abilities at negative ki, and at any moment hyperarmor out of stagger immediately into any of those abilities, you need to stand at range and bait out an attack, punish him with 1-2 hits, and then repeat. He also input reads in a far more egregious manner than most enemies do, so counters like Moon Shadow just aren't going to work unless he's stuck against a wall.
That strategy works for most of the harder human type bosses, simply treat them like O&S and bait them into an attack while retreating, punish them, and repeat. Eventually you're going to start fighting multiple bosses at once, and again it's largely reduced to just backpedaling and fishing for an opening where you can get a hit in on one without being instantly killed by the other.
Of course you could always go hard Dex and fairly effortlessly delete bosses from range with kunai. Ninjitsu in Nioh is Sorcery in Dark Souls 1.