From my limited experience I would say focus on 2-3 girls ideally of different type (melee/range/mage-support)) is the way to go. I am currently stuck because I spread my resources too much especially among melee so now it will take me a long time to build at least one character up enough to be able to be at least remotely useful in lvl 30 raids or be able to clear lvl 30 event maps with ease. Sure you will get there eventually but you will waste quite a bit of time. I am glad I tried it in pretty much pre-release state for most unless you use vpn. It has been a good learning experience if I ever decide to play it when it is officially released.