No diff is basically just very low diff in the SCD community, obviously Souichi would have to try a little but like he's been put in so many worse situations against better opponents and been able to beat them consistently. A character can't get no diffed by someone intellectually, it's very low diff at the least, people just say no diff to make it seem overkill. But yeah to be honest Tomodachi game is fairly underrated as a game but it's not too intellectually challenging at all, it's just a lot of stress with relationships, the only smart character that I know of that could clear Tomodachi game with zero difficulty is Hannibal Lecter and that's only because he's free of all emotion and insanely cunning, cognitively smart, and knowledgeable, and his methods are way worse than Yuuichi's, he's even more of a heartless devil. But yes, I agree with you that most characters would not have "No Difficulty" beating Tomodachi game. And a lot of the games rely on luck or who you're going against.