
Originally Posted by
Tavrobel
Since you have Yuna, bust out Holy as often as you can (I don't have this option when I play). Using Tidus, cast Haste or Hastega on all of the members who can do large amounts of damage. Auron should just attack normally. Tidus can either use Quick Hit, or substitue for someone who can do more damage (most likely Kimahri or Wakka).
Since your seconddary characters are weak, this will be much harder. If Rikku knows Spare change, throw 99990 gil to cause 9999 damage to Sin. Kimahri should use an Overdrive, and then substitute for anyone else (preferably Mighty Guard if you have it). Lulu can Fury any high power spell (Flare if she has it, otherwise, use a level 3 elemental spell). If Lulu also knows Doublecast, then keep her in the party after you recall Tidus. If Wakka knows Attack Reels (not bloody likely), use it, and hope you get 3 of the 2xHit tabs for massive damage. Otherwise, line up three of the same element to cause something in the area of 9k damage.
Sin can be rather unpredictable, so make sure you can heal status ailments quickly (especially Petrify), and get everyone back onto Haste if you lose the buff. Fortunately, this Sin is rather passive, and uses few heavy damage attack options. Rikku can heal most of these ailments causable by using an Al Bhed Potion.
If Sin's HP is getting low (or his Overdrive is near completion), then bring out an Aeon (Bahamut, usually), and hope it finishes off Sin, as Aeons charge Sin's Overdrive Bar EXTREMELY fast. Using this strategy (which is based on a blitz of pure power, as the battle is all about that), you should be able to prevail as long as Auron can do more than 2000 damage, and Tidus can do more than 1500. Holy should be dealing at least 5k damage, but the serious drawback is that Yuna's Holy has one of the longest cooldown rates in the game and is expensive, which compromises her surprisingly excellent speed.
Remember, you can save in between the Fin and Core Sin fights and the Overdrive Sin fight. Make sure your characters have the best equipment that you currently have, and that Sin is immune to almost every ailment.