Others have already said things about the rest, but this one:
Quote Originally Posted by Kazuto9x View Post
3. Do I even need -50% debuffs with elemental advantage? Because I don't think I will get for each element a -50% debuff...
Need? No. But you should absolutely aim for it, yes. Thanks to Vine (acquired from Exchange Shop) and Yatagarasu(/Jormungand from Eidolon Orbs later), everyone has access to -50% Def and Atk. You need one (1) Hime with a -20% Ability and you're set. Your Soul can cover the rest. However, this still leaves you lacking in Black Propaganda, so having at least two Hime with debuffs would be highly recommendable.

This does, however, prevent you from some strats such as Shingen builds. And this is where the team building aspect comes in.