Get the Celestial Weapons, its a must. If you have Tidus in your party, you must have picked up the Sun crest when you first had your chance, you wont be able to get it without beating Dark Bahamat now. Who is very hard. Make sure your wearing armor with Ribbon, Auto-Haste, Break HP - Limit and Auto Phoenix. You should use Quick Hit as well, its very useful.
Dark Valefor and Dark Ifrit are alot easier than the others, so be more prepared then you were for Valefor and Ifrit. Oh and use a guide, I used one for Stat Building and beating the Dark Aeons.