[quote=Theobromine]
The demons seem amazingly strong though. Would there be like an Achilles' heel sort of deal or do we go RPG-style and try to level up before we face these big bosses? xD
[/quote]

They all vary, actually. There will be some that are unfair, but have insane weaknesses. There are some who are fair fighters, and truly require skill to fight against.

Some will literally kill you in a few hits if you meet them alone. And require some serious teamwork. Or massively great strategies.