Ok, 1st of all, stop wasting your money bribing Malboros... go to the Arena is the calm lands, and fight original creation "Shinryu". However, before doing that, try and get Wakka's and Tidus' Celestial Weapons, powered up, and have Wakka's Attack Reels, and Tidus' Blitz Ace.
Then, when you beat Shinryu, you get at least 1 (2 for overkill) Wings of Discovery.
Dark Valefor then becomes REALLY easy. Get Wakka and Auron with their Celestial Weapons, and Wakka with an Overdrive.
Auron's weapon allows for "1st strike", so, switch out Auron at the start of the fight, and switch in Wakka. Then use Wakka's "Attack Reels" and Dark Valefor should take considerable damage, making him at least 80% dead, if you hit all "2 Hits".