Nioh - Toyotomi Hideyori - Way of the Nioh (No Items, No Ninjutsu or Onmyo, No Living Weapon)

The healing comes from receiving an elemental debuff, (ticking healing) or landing a perfect guard (800~ heal.)
Around 50 attempts for Hideyori. A unique boss, changing from human to yokai in his 2nd phase. The discord debuff is a definite threat in this fight, a lot of elemental damage being slung around.
It's important to keep in mind he gains infinite armor when he changes into his yokai form, so you can't swing as recklessly as when he's human in the first phase. I enjoy this boss, even though he does tank my frames pretty hard.