Instructions
Sin's Left and Right Fins
Make sure Tidus, Auron, and Wakka are in your party, and command Cid to move close to the left fin.
Have Auron use the "Armor Break" and "Mental Break" maneuvers on the fin, and have Cid move away from the fin. Replace Auron with Lulu.
Have Tidus cast the "Hastega" spell and begin attacking the fin with Wakka's basic attack and Lulu's level 2 black magic spells ("Fira," "Watera," etc.). Repeat this process (remembering to heal when needed) until the left fin is defeated.
Repeat steps one through three when you move to the right fin. Once the right fin is defeated, your ship will move on top of Sin and your group will jump onto his back.
Sin and Sinspawn Genais
Make sure Tidus, Wakka, and Auron are in your party. Take turns attacking Sinspawn Genais until it enters its shell.
Replace Auron with Lulu when Sinspawn Genais enters its shell, and have her use fire spells on it ("Fira" or "Firga") until it is dead. If it comes back out of its shell, return to basic attacks.
Focus on Sin now that Sinspawn Genais is dead. Replace Wakka with Rikku. Replace Lulu with Auron and use Auron's "Armor Break" technique.
Have Tidus cast "Hastega" and keep attacking Sin with basic attacks until it is defeated. If Sin casts the "Negation" spell, perform "Armor Break" and "Hastega" again. If your HP gets too low, have Rikku use "Al Bhed" potions. After Sin is defeated, your group will get back on the ship and move to Sin's front.
Overdrive Sin
Start with Tidus, Wakka, and Lulu in your party. You will be far away from Sin at the beginning of the battle.
Have Tidus cast "Hastega" and replace Tidus with Rikku. Have Wakka attack Sin with basic attacks. Have Lulu attack Sin with level two or three black magic. Have Rikku attack Sin with the "Use" command.
Replace Rikku with Tidus and Lulu with Auron when you are close enough to Sin. Have Tidus cast "Hastega," have Auron cast "Armor Break," and attack Sin until it is defeated. Remember, you only have 12 turns until Sin can automatically destroy your party.