BOSS ANALYSIS : With Dark Valefor you know what to expect from the offense. There’s a basic Attack, Sonic Wings (damage + Delay), and both of his Overdrives will make an appearance, each dealing damage to all. Energy Ray can be halved with Shell, but Energy Blast you’ll just have to take.
MAIN STRATEGY : Having Protect on from the start will halve most of the damage you take, and Haste ought to give you 4~7 turns before Valefor gets a move in. Instill whatever you don’t have Auto’d among those with the appropriate spells or Super Mighty G. Auto-Life can be valuable if you don’t trust your Aeons with cannon fodder duty, but otherwise not - you should be able to survive or “cannon fodder” your way through everything.
Your offense is pretty simple, at least - lots and lots of Quick Hit. Quick Hit is about the only way to deal damage, since it is on average 83% more powerful than magic (taking Valefor’s Defenses only into account). Beyond this, heal when needed (though Auto-X-Potion ought to suffice) and send in Aeons to block for Valefor’s Overdrives when his gauge is maxed.
ALTERNATE STRATEGY - OVERDRIVE ABUSE : This is simple enough. With Break Damage Limit and high enough Strength (or Magic and maybe Trio of 9999 with Lulu), you are able to do massive damage to this boss in a small number of turns. In doing so, you must first enter the battle with as many Overdrive gauges full as possible. Have your main Overdrive user use an Overdrive, then have others Entrust their gauges to that person and keep attacking until done.
Question is, who to use? Tidus has the most reliable one in Blitz Ace, hitting 9 times when done right. Wakka has the luck-based 2x-2x-2x Attack Reels, which hits 12 times. Lulu hits the most if she uses one of her low-level (Fire, Water, etc.) spells with Fury, hitting up to 16 times, but it’s more likely 8 if you have to use Flare or Ultima. It’s really up to you but just use them all and use your Aeons too.