HONESTLY I want Mephisto to get rekt if only because he can recover from it and his ego can always use a little knocking down. He can get shot in the face and literally walk it off, so he's pretty sturdy.

Maybe they could run into a guy who can control people with his mind? But with the caveat that he can only control one person at a time, so he tries to force one of them to fight the other. He probably wouldn't be able to force Mephisto to use his spoopy demon powers, but he'd still have his super strength to worry about (not that the guy would assume Mephisto can do that, but he might think that Jae-ha would be less likely to hit someone who doesn't look very physical strong).