you're forgetting the 're-awakenning' part.
most start between E-A level, S-ranks are those who got re-awakened and had mana increase. it's stupid.
so the tiger beast guy was probably hand-to-hand fighter, maybe a little but of transforming, but then got a power boost. same with the rest.
power (mana) and the abilities are set in stone, but you can still get better at fighting by trainning, being in better physical shape, putting yourself in the correct mindset, learning tactics, working together with others... the skills that are actually intresting to watch.

isn't jinwoo being an edgelord chuunibyou asshole that sucks kinda the entire point? the whole point of how he behaves is supposed to make the audience dislike him, isn't it?