Hopefully you've beat him by now, but my sister just got to this point in the game and that wasn't good advice, this is what I did.
I pulled back first off, in order to avoid his poison attack, then cast haste on Tidus, Lulu, and Wakka, cast blizzara, dark buster and just have Tidus use cheer on his turns, continue this until he has about 15000 hp left an move in, now you'll have to deal with poison, but you'll avoid his swooping attack, as soon as your closer to him have Tidus use Delay Attack as much as possible, That worm will cast haste on itself even when it's not his turn so don't worry about getting rid of it, just keep using delay attack. My sister had megaelixer's so if at least 2 people were yellow I would use them. Well that may be an odd way of beating him, but it worked after about 10 fights and then my poor sister died right after, so she's going to have to fight him again. <img src="/images/graemlins/frown.gif" alt="" />
