Sin Overdrive!

[~Megalixir~]

First, level up Tidus, Wakka, Lulu, Yuna, and Auron. Do random battles until these characters' OVs are up. Set Tidus, Auron, and Wakka's Overdrive type to warrior. Then, make the opening party Tidus, Lulu, and Wakka. Finally, make sure Tidus, Wakka, or Auron has Sensor.

Use Hastaga with Tidus in the begining, casting (or Double-Casting) spells with Lulu and hitting Sin with Wakka. When in close range, Switch the party to Tidus, Auron, and Wakka, and cast Hastaga again, then hit it with attacks and overdrives as hard as you can (don't heal unless a character is dead. ) When Sin is down to about 15000 hp, switch one of the characters for Yuna and use Grand Summon> Bahamut. Use Bahamuts Overdrive, and thats it. (if you made it...)

This may not seem the best way, but you dont have to spend hours leveling and getting the Magus Sisters, and geting Double-Cast now for Lulu takes some serious gameplay (I had Double-Cast with Lulu).