For me HDR did fix it. I had the exact same problem on my PC monitor before I enabled HDR in Windows which made it detectable in game. I could barely see anything at all and would have stopped playing because of it. Looks real good with HDR on too Okabe Is Sometimes A Good Bean ...
Unfortunately Nioh 2 natively has a bug where the UI drifts after certain events at any aspect ratio above 64:27 (2560x1080). There is an EXPERIMENTAL fix that and also allow the game to have fully visible UI and not stretched at aspect ratios below 16:9, but both fixes come with cons...
The 2 Nioh fight is easy without the living flame, use the Bull Guardian spirit and it knocks the big guy down every time, 3 times with carnage etc and its all over. The Ice Lady / Living Flame guy are a little harder, but sloth/carnage/weakness, with the Sword mystic art and get ...