What are spells, what do they do, and how do I get them?

Also, being that you can only select 2 weapons, why do you need a battle set?

All the answers to your Spells Questions are located Here!

And as for the sets, you will have to use more than just 2 weapons, But only 2 at a time, Some are once per battle, Some are healers, Some are blockers, Some are freezers, So you wont have to use those same 2 weapons, its a strategic thing xD

Awe ok, because I keep using my legendaries and it seems like I never use the rest of my (crappy) weapons. Thanks for the link :)