Alright, hear me out. I know some might not see it the way I do, but just listen.
What if you could coat your sword with the wisteria poison, which specifically affects demons?
Some of you might say, "Oh, Shinobu can already do that." But you're missing the point. You're too predictable.
If the poison is applied to your sword, it would last a solid 20 seconds, and any hit would poison the target for 5 seconds, dealing two points of damage per poison tick.