Skip ahead to 1:55 to bypass the dialog if you want.
This is a pretty much a story battle and you can't lose unless you try, even so I left it in anyway cause I like the scene. Plus you get to see a fully powered Shiranui. The 12th fight is a story battle against the Silver Demon, Nechku. This strategy is based on using Amaterasu's powered up Thunder Edge as a main weapon and powered up Eighth Wonder as the secondary (and the skill that increases damage when two Glaives are equipped). As with my other boss videos, my strategy isn't absolute and I wouldn't mind other effective strategies or advice posted as comments.
For the fight, even if you don't do anything Shiranui will pretty much win this for you. But if you want to try your luck, Nechkku will shoot projectiles which you should hit back. Also Shiranui will also use brush strokes on your scroll if you have it open.