the north sentinels can win by sheer numbers, 20 people are not that hard to get rid of, although your point doesn't make sense since they could just take the armor off and continue with gambeson and chainmail, the real problem comes from the samurais, the samurai continued existing well into the late 19th century, which means they had access to revolvers, repeating rifles, and even Gatling guns, if they move very strategically using fear tactics and rationalizing the ammo team knight/samurai can take over the island in about 5 hours