I did it just like I said I would earlier, the only change is that instead of relying on two misses, I brought Tidus and Wakka back to life, that way you only have to hope he doesn't kill Rikku, if he kills Wakka or Tidus, you win, because you can Trio of 9999 and then finish him.
My battle today went like this:
>>Enter Tidus, Auron, and Wakka
>>Sanctuary Keeper uses Photon Wings
>>Auron lives and evades sleep and confuse status
>>Auron switches to Rikku
>>Rikku uses chocobo feather
>>Sanctuary Keeper physically attacks Rikku (misses)
>>Rikku uses phoenix down on Tidus
>>Rikku uses phoenix down on Wakka
>>Sanctuary keeper uses physical attack on Tidus (dead)
>>Rikku mixes trio of 9999
>>Sanctuary Keeper uses Mana Breath on Rikku (dead)
>>Wakka uses Attack Reels (Overkill)