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Connor McLeod has near-invincibility going for him, but if Musashi knows how to kill him, he can do it.
First of all, I doubt Connor would tell his opponent how to kill him. Part of his power lies in his secret: Connor is an ordinary looking guy who just happens not to die unless he's decapitated. That's just not the sort of thing Musashi has trained to expect when he designed his fighting style.
Musashi will probably die before he figures it out. Remember, the Japanese swordmaster's philosophy: when you commit, put everything you have behind it until you win. Connor has literally centuries of training: he can fight in Musashi's league, and would cut Musashi in half once Japanese blademaster committed to an overwhelming attack that would have killed anyone but Connor.
Even if he knows Connor's weakness, it won't be easy.
From the Japanse sword techniques I've seen, no swordmaster, let alone Musashi, focused their training on a horizontal cut at head level: it's just too easy to block.
Japanse swordwork emphasises cutting the skull in half, slicing the body diagonally to shatter the collarbone, cutting up the hands and tendons, etc. None of those work on Connor.
Connor McLeod just has to hit Musashi anywhere, and Musashi will bleed until he dies. It won't take long.
Musashi has to land powerful killing stroke at a target he isn't used to aiming at, against an opponent who's trained for centuries to block that very strike.
Musashi's victory is anything but guaranteed. I say he'ld be sliced to ribbons.