Leave it at F for now unless you are considering being a mage or spamming bolt fusion. (I don't suggest being a mage in this game though, end game will troll you very hard because everything and their mom has level 2/3 mana deflector). r1 icebolt isn't all that great either, the mp efficiency gets thrown out the window for like some 70 damage. If you get a chaincast wand it effectively becomes a machine gun though, which is useful against normal mobs.
I still suggest staying warrior destiny to rank windmill, because that is a pain in the butt to rank when you have high CP (just ask me...3000 zombies just for awful kills wheee)