maybe I'll actually help the guy?
I couldn't get past the dark magus sisters in X-2 either, and you should have seen me first fighting jecht 0_o.
ok, fighting jecht:
get 99 stamina tonics by caputring all the fiends in Besaid, and chatting to the monster arena guy, they'll make the 3 people you have out at the time you use it hp go to 9999.
2nd: wait untill phase 2 to use Tidus's "talk" comand, as it will make his overdrive gauge go back down, and you dont wanna be faced with his 2nd form overdrive, not really pretty
3rd: You'll wanna get slow on the Yu-Pagoda's, other than that, ignore them. Jecht is suseptable to zombie, so when the Yu-Pagoda's heal him, it will damage him, but only that once, cause they cure his zombie, dont go out of your way to get it on him but.
4th: the ultimate weapons are always really handy, but if yuo dont have them, zombie strike, 1mp cost and stone proof are handy to have. (you dont want a party member eliminated at the begining of the battle with jecht beam)
I didnt use Aeons in this battle at all, but they do help when nearing the end of the battle to polish him off. Before the battle, flee for all other battles (duh)
Hope i was of SOME assistance Blackraven19