1.You must at least maxed out your accuracy, defense, and attack
2.Customize your armor with Auto-haste, Break HP limit, Auto phoenix, and the last thing is Ribbon (This is the most hardest part, you need 99 dark matter! )
3.ALWAYS use Quick attack!!normal attacks are too slow and not recommended at all,ok ?
….I suggest you to use Yuna, Tidus, and Wakka.
Yuna: help other character with Protect first,and shell
Tidus & Wakka : Always use Quick attack (change your mode to warrior ) and use their limit (Wakka would be very use full if you can use his attack reels correctly )
I think that’s all.
maybe other shriner has another tactics, so be patient. ^_^

Dark Aeons ONLY available in FFX international.
[ Spoiler !! warning! ^___^ ]
The dark Aeons are ultra hard versions of your normal Aeons which become available to fight during the last part of the game. I am not sure of the exact timing but i started fighting the dark aeons only after reaching the last save point just before the final boss inside Sin.
Do you know understand now?
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And it’s as easy as that.