I read another version about Musashi. That is, he made a wooden sword/boken from a paddle, it's more like a spike/spear than a sword. He knew he can't match the reach of nodachi therefore doesn't use the katana. Of course he didn't want to clash "swords".
Musashi then anger him (being late and taunting him) and play trick on Kojirou with sunset's rays.
In the end Musashi won by a single stab on Kojirou's lung, that eventually killed him.