auto-phoenix will help in some of the battles, but remember you will only get back a maximum of 9999 health... sometimes it is better to do full life to restore full health. however, i suggest putting it on one of your 3 characters.

i say scrap auto-shell... you dont really need it... in fact, auto protect is probably better because i found that the dark aeons do quite alot of physical attacks.

you will DEFINATELY need auto- haste on at least two of your characters, because otherwise the dark aeons will get to many turns in, and you will get pulverised.

lastly, if you can get ribbon, it is definately worthwhile, but you dont need it for the first few dark aeon battles

(btw, you will probably win some pretty cool armor in the first few battles against dark aeons, so if you find the battle easy, and you dont get any cool armor, i suggest reseting until you get an armor with ribbon as you will definately need it against dark bahamut, anima, yojimbo and magus sisters).

i used tidus/wakka/a weaker character (still with ribbon) for most of the battles, and brought yuna in to summon when their was an overdrive. the purpose of the weak character is to get fast overdrives when they get hit (so you can use attack reels over and over again).