@Operatix However he still ends up losing despite his final form, and his final words are,
"Kukuku. Alas, I am but the weakest of the four. You shall never make it to the demon king!"
Except that there is no demon king currently as the sealed girl is the daughter of one of the former demon kings, has inherited the powers to become the next demon king and that demon planned to eat her to obtain that power and become the next demon king himself.
@EmptyNight
You're right. But I first went into this thinking it was a seinen manga (something catered to young adults in their teens to early 20s), as that's what it was listed as. But fighting-specific types are usually categorized as shounen with all the usual tropes (catering more to adolescents who like mindless action) as that's more of their speed ¯\_(ツ)_/¯