I think I tried the Platinum match as soon as it was possible to enter it, and while I lost the battle without causing any major damage to Sephiroth, I was still able to survive for some time (enough to hear the whole music once, I think). Probably choosing the shield and giving up the sword in the beginning made me endure his hits longer (my brother chose the sword and gave up the shield, and when I was helping him with bosses, he took a lot more damage than I did), and using healing spells whenever I was badly hurt helped me too. Sephiroth has never been able to finish off Sora with one hit in my game.