Sanctuary Keeper uses several different attacks hitting one or all of the party. Photon Wings causes Darkness, Curse and/or Poison and/or Confuse as well as damage. Mana Breath is directed at one target, but inflicts heavy magical damage.
It will often use Tail Sweep, which delays the turn of all characters. After suffering enough damage, Sanctuary Keeper begins to cast Curaga, restoring 9,999 HP. Sometimes, it will cast Reflect on a party member, then cast Curaga on the same party member so the heals will bounce back at it. If afflicted with Slow Sanctuary Keeper counters by casting Haste to nullify the Slow, but if it has Reflect the spell will bounce onto the player party.
Even with a weak party, fighting the Sanctuary Keeper is easy as long as the party has Dispel and Bio/Poison Fang. At the start of the battle, one should cast Bio/use a Poison Fang followed by bestowing Reflect on Sanctuary Keeper. With Reflect, Sanctuary Keeper is less likely to cast Curaga.
Auron should use Power, Armor and Magic Break, then use physical attacks while Yuna heals. Sanctuary Keeper's Photon Wings causes various status effects, but using Remedy heals Darkness and Poison, and a Holy Water is needed to cure Curse. Auron and Yuna are the most helpful characters in the battle, though Lulu can help by casting Bio and Rikku can use items for various effects.
Yuna should buff the remaining members with Protect and Shell statuses and cast Reflect on Sanctuary Keeper once it is poisoned. Rikku can also bestow buffs by using items or via her Mix. Shiva is helpful as the Sanctuary Keeper is not immune to Heavenly Strike. Repeated use of it allows Shiva to keep attacking without risk to the party or counterattack from the Sanctuary Keeper.
If the party has a Silver Hourglass Rikku can use it to inflict Slow on Sanctuary Keeper. This prompts it to cast Haste but it will be reflected onto the party. A Smoke Bomb can be used to inflict Darkness to dodge the Keeper's counterattacks that hit the entire party. The party can also try using Threaten on Sanctuary Keeper.