I think you mean the Sanctuary Keeper which you encounter in Mt. Gagazet. To beat him you should equip your party against Sleep, Silence, Dark, Confuse (I hate that one!) and Curse before the battle as these will be inflicted by the Santuary Keeper's Photon Wing. It makes it easier to beat him if you cast Hastega, Armour Break, Mental Break and Dark Buster at the start of the battle. Using Protect is a good idea, as this will reduce some of the damage that you receive. If you decide to use an Aeon to help defeat him make sure they are ready to unleash an Overdrive attack as soon as you summon them. This is because shortly after you summon a creature the Sanctuary Keeper will use Mana Breath and your Aeon unfortunately doesn't stand a chance against this! So as soon as you summon an Aeon, use their most powerful attacks (which are Overdrives) straight away. Mana Breath also causes around 4,000 HP of damage to one of your characters, luckily it is only the one and you can easily revive them. The Sanctuary Keeper occasionally uses Curaga to boost his HP by about 9,999 points. You can prevent this by casting Reflect on him or have one of your characters with the Use command use a Star Curtain on him. If he casts Reflect on any members of your party then you can reverse this by using Dispel or swapping them with another character. Finally, the Sanctuary Keeper doesn't have any weaknesses or strengths with any of the elements so use whatever attacks you think are the most powerful, his HP is 40,000!! You can steal Turbo Ether(s) or if you're lucky a Return Sphere off of him! Hope this helps, good luck! :-)

The easiest way is just to cast reflect on the boss. His curagas will bounce off and cure you.

the easiet way to defeat this boss is to have all your aeons overdrives ready from the fight in the cave. In the fight bring out all your smaller aeons except bahamut get his hp under 20,000 then summon bahamut and do his overdrive. remember when casts reflect on you cast dispel on that character right away also remember do not attack him physically with any charachter because he counter attacks with whole lot of curse,poisen and more. Just your aeons.

Firstly have Yuna, Auron and Tidus on the field (or your strongest characters) and make sure Auron and Tidus are close together when Yuna is as far away as possible, (before the battle equip your characters with berserk ward armour) then just keep attacking from there but here is a little help on what the characters should be taking away: Auron: between 3000 to 4000 Tidus: between 2000 to 3000 Yuna: Just make sure all her Aeons have there overdrive gauge to full and summon valefor first then Ifrit, Ixion, Shiva then bahummut. P.S- It worked 4 me, hope i helped.

i beat him by shoving a stick through his throat then making yunalesca summon the final aeon to finish him off hope i helped