So like, I think the fact Arthur has weapons is something that is not being taken into account enough, even if Little Mac is more skilled the Arthur, one needs to acknowledge the range gap between them. Arthur is bigger and has a sword, Little Mac is only 5'7'', on top of that, Arthur has a shield, which is a much better block tool then arms.
Sure, Little Mac has fought bigger opponents, but that's still not comparable to someone who has a sword and a shield, stamina hardly matters, the fight is not going to extend for too long before one of them loses, and even if they start grappling, Arthur should, once again, have the upper hand.
Like, unless I'm missing an important detail, I don't see how Little Mac wins, they don't even need to fight to the death, if Little Mac loses a limb, Arthur might ask for him to give up, and I don't know if a boxer would continue fighting after losing an arm if given the chance to give up