SIN (HEAD)
HP: 140000

STRATEGY:
While Sin draws you in, cast stuff like Hastega and Cheer (Don't bother with protect as you rarely get attacked). Once you are close enough for combat, hit him with armour break and Quick Hit. Once he is down to around 100000HP, you have two options:

HAVE ANIMA: Summon her and use Pain over and over. Casting haste on herself would also help. Right at the end, use Oblivion to finish him off. NOTE: Do NOT use any aeon overdrives before the end, as it takes too long for them to recover.

DO NOT HAVE ANIMA: Use attacks like Self-Destruct and Spare Change to bring him down. Once his HP is low and his overdrive is almost full, summon Bahamut and send him back to hell with Mega Flare. Alternatively you could unsportingly use Yojimbo.

Another way is to use Trio of 9999 (Mix lvl3 or 4 key sphere with a wing to discovery, underdog's secret, or pendulum)