You described 90% of Shonen's less the part while doing it they face opponents far intelligent, experienced... well you get it, anyway he is inside of one of those Shonen, so or win by yourself or let it to the protagonist because he will win anyway... it is "just dumb" but that it a Shonen in a nutshell, under dog kid wins op villain who everyone on the verse couldn't.

that was disappointing, nobody needed him almost like everything would work out even without him there... i mean he didn't even win, next time he should learn from vegeta and wait for the protagonist

he could use a bet to regenerate or seimei kikan (is TO Strange that his df don't offer protection aganist this 'sickness'... just imagine Magellan being poisoned by his OWN venom or enel being struck by lightning)

This thing about creating your own enemies, most heroes do that, flash-zoom, batman - 2 faces, batman-joker, superman-Zod, iron man - iron monger, the list goes on forever... honestly most heroes do more harm than good, apart from planetary events, universal or multiversal of course.

future superman is my favorite one