The final boss took me close to 2 hours. Reloading the game today, it started me back at the humanoid boss before the main final boss. It worked out well since I got to the final boss with alot more health than my previous checkpoint. I was able to break through that first form on my second attempt today. That first form of the final boss is a huge pain in the ass. The idea is to use the Lunar/Staff and charge onlanding UT's on the left edge of the arena, set the camera to the other option (prevents camera whipping), after landing a staff UT immediately hit Y to follow up with a delimb finisher. Rinse and repeat (make sure you run away and have enough distance between the enemies to charge UT safely). This builds the yellow ninpo bar which you must max out to unlease the Ninpo attack which defeats the first phase of the boss.
For the final main phase of the boss (forget the QTE's) , I found that I could get up in the corner of the arena and use the Scythe to land regular Scythe attacks on the boss's elbow again and again. Aside from that I just block and wait for the elbow to be in position. Just make sure to get out of the yellow warning area for the final arm slam, as it breaks the building (died atleast twice on that cheap bullshit). I got my info for the first form strategy from checking various forum posts and videos, really fuckin obscure bullshit fight. The second form I figured out that strat, but that form is not hard. Might be better for speedruns though as it seems faster then what I saw the speedrunner doing for that part.
This is from Ninja Gaiden Master Collection on Series X. Full unedited playthrough coming soon.