mmk, so if it's the one I think it is, here's a few pointers for you. This strategy basically works for every boss in the game too, in case you have trouble later.

First of all, do you know how to cast a healing spell on all three members of your party at the same time? If you dont' play with the controlls a bit, i'm not exactly sure how to explain it. But anyways, set Rikku to white mage, and set Paine and Yuna to whichever dressphere has the highest strength (I think at this point, it's warrior. But if you want to put one on warrior and one on gunner, trigger happy works nicely.) So cast cure on all three people in your party at the same time every time Riku has a turn, even if you think everybody's okay. Then basically the other two hack and slash at him untill he's gone. If you are using warrior, the Excalibur ability works really well if you have it. Tah-dah!

Hope this helps!