Sure Mifune is a strong opponent and is a great sword-master. But I have a theory on how to defeat him. Here's how: Mifune is a master of Iaidō. Iaidō refers to a specific style of kenjutsu that is concerned with the smooth, controlled movements of drawing the sword from its scabbard, striking or cutting an opponent, removing blood from the blade, and then replacing the sword in the scabbard. The linear motion and force applied to draw the sword from its scabbard result in much faster slashes than ones realised with the sword already drawn. This sounds familiar doesn't it? It's the same as Battōjutsu from Rurouni Kenshin. The secret behind this a great control of the sword and also this explained during Kenshin's battle with Udō Jin-e: Battōjutsu is achieved by placing the sharp edge of the blade up against the curve of the sheath and going with the curve increases the blades drawing speed. However, with such cases as a sakabatō, the speed is reduced due to the dull edge of the blade being up against the curve of the sheath and so it's not as smooth. Therefore, I would probably use an oxidation jutsu (you know when metal comes into contact with oxygen it slowly begins to rust) speeding up the process making Mifune's blade dull and rusted. I would achieve this by crossing swords with him and channeling this jutsu through his sword. And then his technique would be brought down, and I would have the advantage since Mifune's attacks mostly rely on him using his katana.