Hey Sky High,
you know the attack that did 9999 damage to your whole party, i hate to tell you this but that ain't his overdrive , D. Valefors overdrive does 50 000 or so.

Heres how i did it (or was planning until Yojimbo Zanmoto'd him )
Make sure all of your characters really strong, try over 150 (once you reach the hundreds try fighting him every now and then) in all departments to be safe. Get all the secret aeons and power them up (and i mean POWER THEM UP), try to get them in over 200 in each department. Have all of the Celestial Weapons, all of them.

Now that you have done that, if you have a good strategy is like this :
you should have the first turn with your agility. What you should do, well, what i did was this (these were my characters and status and stuff).
have auron attack (hopefully with an overdrive if you have it). THen was Yuna's turn in which i summoned an aeon, i chose Anima first. Get her to attack until she dies. Then should be Tidus's attack, have him cast Hastega. Try to keep your whole party alive using Tidus and Auron (yes, for once they can be a healer), and whenever it is Yuna's turn summon, you must summon since Dark Valefor is very strong. In order, summon Anima, Magus Sisters, Yojimbo, Ifrit then any after that. If you can't beat it you may need to strengthen up a bit more or you could also try a different strategy.

Hope i helped.