Quote Originally Posted by Sora View Post
Furthermore I tought that I don't really need Mordred and D'Artagnan when I can just use Dark Amaterasu.
Correct. You are in a rather unique spot with that, as Dark Amaterasu is not a common thing to have.
I'd still highly recommend grabbing Mordred at least once you've taken care of the rest of the useful Souls (Gawain, Cass, Joan, maybe Andromeda). Because this way, you can do content with other element teams while leaving out Dark Amaterasu. That might be a thing you'll need to do one day... ... though probably not since Dark works against all elements.
Quote Originally Posted by Sora View Post
Can someone explain this "grid" please? Sometimes I read that some people have a 120% Water grid or 150% dark grid?
How do you calculate this %?
Sadly, there's no easy way to calculate this.
R weapons are 0% base. SR weapons are 3%. SSR weapons are 6%. Dual skill weapons, if they're Hime release weapons, usually have one SSR skill and one R skill, and event dual-skill weapons are SR and R.
Each skill LEVEL is 0.5%, and yes, that means that dual-skills double-dip into this.

So for example 10 equipped SR weapons with each slvl20 is (10*3%) + (10*20*0.5%) = 30% + 100% = 130% grid.

I've made a spreadsheet for myself to help keep track of what weapons I have for each element and their skill levels, and it calculates the exact percentages for me.