The Sanctuary Keeper is a boss from Final Fantasy X. Fought at the summit of Mt. Gagazet, it is a hulking Sacred Beast fiend sent by Yunalesca to challenge the summoners and their guardians before entering Zanarkand.
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; and 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.
Even with a weak party, fighting the Sanctuary Keeper is easy as long as the party has Dispel, Reflect and Bio. At the start of the battle, one should cast Bio followed by Reflect on Sanctuary Keeper. After casting 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; using Remedy heals Darkness and Poison, but 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.
Yuna on the other hand should buff the remaining members with Protect and Shell statuses while she should cast Reflect on Sanctuary Keeper, allowing any other party member to gain slight benefits but only do this once Sanctuary Keeper is under Poison status ailment.
Shiva is helpful as the Sanctuary Keeper is not immune to Heavenly Strike. Repeated use of Heavenly Strike will allow Shiva to keep attacking without risk to the party or counterattack for the Sanctuary Keeper.