you have to hit him with the magic that matches the same color of his head or something. And to slow down the death timers use stop on the big clock. Make sure to bring plenty of ethers too.
Yeah, for thunder, you can just take a pot shot and most of the time you'll hit. It was even easier in KH1 because there were a ton of little ones that just rained out everywhere.
Really? My favortie boss fight had to be Kurt Ziza because it took so much strategy and skill. With Sephiroth, all you really needed were lots of elixirs and really fast fingers.