You don't need stronger muscles, you just need fear
The difference in strength between the strongest and average human is really not that important in the scheme of things. Especially in sword fighting, even the most compact muscles can easily be punctured by a blade. In a room full of swordsmen, a lucky strike can shear through to bone, and then the master swordsman is done.
On the other hand, remember that scene from Unforgiven? You might say it is patently impossible for one man to kill 7 in a shootout without being hit back. But that is explicitly discussed in the movie. Kill the leader first, keep up the attack, rely on fear and a bit of luck.
Using swords and wearing armor makes this even more possible. Fearful strikesStrikes from a fearful arm lead to glancing blows. The intimidation factor of the man next to you being shot is nothing compared to the man next to you erupting in a fountain of blood as someone cuts his torso nearly in half, etc.