Which Sin? If it is Overdrive Sin, all you need is holy, one aeon, some eathers, doublecast, and Auron. Auron does all his Breaks. Lulu does doublecast flare or ultima. If you are unlucky like I was and didn't have Flare or Ultima, use Blizzaga. That was good. Anyway, use Wakka until Sin draws close enough so that everyone can attack. I had a disturbance so I couldn't think one of the times I fought Sin. "That's a rock!" "No its not, its a blitzball!" "There's no such thing!" "He's a survivor!". Family. You could do without them. Anyway, after Sin is close enough, use all your overdrives. It will help so much. Use one of your strongest aeons for the overkill if needed. The guidebook is wrong. The overdrive goes up if you use an aeons overdrive and thats why you use one for thr overkill. Throw 100,000 Gil to do 9,999 damage points. Yuna's Holy will do 9,105 points if you are leveled like I am. All your strongest players should have 4000 or more hp. Wakka, Auron, and Tidus were my stongest and everyone else had 3000 and something. 300 or more mp for Lulu and about 280 or more for Yuna. If you don't have their Celetial Weapons, Yuna will only have enough mp for about two Holy's. It costs 80 mp so use it wisely. I actally found it easier than Yunalesca. That's what I did. Hope it works for you...