Ayame : Items : Dokuto blades, caltrops
As soon as the battle with him begins, use the
caltrops and make sure that he step it. Then, strike
him with dokuto blades (crescent moon if necessary).
After he gets up, strike him again and do the same
thing until he died. (don't worry, when he gets up,
he will stagger for a while as the result of the
poison)

Rin : Items : caltrops, your strongest weapon (like
cannon, etc.)
use the caltrops as soon as the battle begins, then
attack him with your strongest weapon.

Have you completed the Red-Blade mode?

Hey how to get the Dokuto blades? I really forgot