When fighting Sephiroth, he will suddenly say Sin Harvest, and a little red thingie will surround Sora. a split second later, it will drain hp to 1, and mp to 0. When Ole Grey Hair says Sin Harvest, go to you items (hopefully all elixers), and choose which item to use, but don't choose Sora, until you see the red animation on Sora. If you use it fast enough, it will activate just after your hp and mp drops. Then, even If sephiroth gets to you, you can survive.