Save Kilika! Hit Sin from a distance with Wakka's blitzball and Lulu's black magic. Valefor's attacks work well, too.
Sin won't actually engage in combat; rather, it shoots out Sinscales to do the fighting while it remains out of range of physical attacks. When the three Sinscales are defeated, three more are brought out to take their place. Players may want to only kill two so they only have to deal with attacks from one. After the Fin is defeated, the player moves on directly to the battle with Sinspawn Echuilles.
Lulu's Black Magic, Kimahri's Lancet, Wakka's regular attacks, and Yuna's Valefor summon can all hit Sin. The only thing Sin actually does when its turn comes up, is that it will pull the ship around for a bit, shifting the camera angle. This can be more than a nuisance, as one may accidentally wind up hitting allies.
Level grinding method
If the player has the patience, they can easily level grind here. Using a party of Tidus, Wakka, and Kimahri, one should have Tidus use Cheer five times to boost their Strength. The three should be strong enough to kill the Sinscales in one hit, and when they die, Sin shoots out three more. With the ability to kill the scales in one attack, the player can kill them in one round without getting hit, or at least not seriously damaged, since Cheer also raises Defense.
The Sinscales each give 2 AP, or 3 AP with Overkill, and as there is an infinite supply, the player can spend as much time as they wish killing them to earn AP and Power Spheres, as well as earning Overdrive Modes Warrior and Slayer for Tidus, Wakka and Kimahri. If the player lets Yuna and Lulu in at the end of the battle, they will earn AP as well.