Basically, you have to hit him hard and hit him fast. Use characters who're quick and strong, and just keep pounding. If you've got special abilities that let you score a lot of hits (Overdirves, Quick Hits, Hastega, Doublecast), then use 'em. However, it's a waste of time using Overdrives that only hit once or twice, because the amount of recovery time seriously outweighs the damage done.

It can help to have the 'Ribbon' or 'Stoneproof' abilities equipped, since one of Sin's attacks can instantly petrify and shatter a party member.

If your characters' levels are simply too low, head to the Omega Ruins (located on the eastern peninsula of the world map) and butcher a few Fiends until you're strong enough.

Good luck!