Level 12 when you leave Destiny Islands? O_o That's going to take lots of patience. I still prefer the Shield to the Sword because you'll still get a good attack power with your Keyblade even if you take the Shield, but when I took the Sword and dumped the Shield, bosses like Cerberus were really beating me up. And with Shield, you get the useful Guard ability very early. Your level doesn't really matter when fighting Sephiroth. The battle is more about skill, IMHO.