Well, it's just a guess from what we've seen, but it looks like you have to be chosen by a demon slayer to undergo training. And typically, he was suprised by MCs grip... not something I'd expect to happen if he's truely a genius.

I kinda liked the fox mask btw... they look cool, I want MC to wear one again, but this time, to actually hide his face. Though I have to say, that enchantment didn't seem to offer much protection.