But doesn't Sam get to use his Onimusha form against Spirit Doku?..
Well here's my thoughts. Reg. Sam against Armored Doku Sam will lose 42%. At their prime I think the probability would be around the same though only a small bit higher, maybe 44%. Spirit Doku's a beast who's intangible (passes through walls) can appear out of nowhere (did it in the fight against Ryu) move really fast and has a spirit form of the kitetsu, which is about the size of Sephiroth's masamune and can drain a shit load of health, and can be tossed and brought back. Doku can throw his hand at his opponent and grab them from a distance. He can also shoot a blue fire that's not ordinary fire, and apparently he can only be affected by spiritual/evil weapons like the True Dragon Sword, as physical objects would have no effect on him whatsoever. I thought I should mention that he brought about the fiendish awakening.
I'm pretty sure Sam's Oni Form is powered up by some form of spiritual energy that can probably be of some effect against Doku.