make your aeons and character overdrives.
it's easy, first have all of your character except tidus, auron, rikku, yuna, which is kimahri, wakka, lulu to do their job so that they earned AP after battle (don't attack just use focus or healing item or just guard)
then change with rikku, auron, tidus. have rikku mix 2 wings of discovery for trio of 9999. change rikku for yuna. have auron and tidus use physical attack to kill the first form. yuna must cast reflect after tidus cast hastega.(you wouldn't want haste to be reflected to yunalesca). (i recommended that you use this white magic before yunalesca enter her second form).
make sure to have yunalesca cast hellbiter to the three of the current party. then simply call all the aeons except bahamut in the battle and unleash their overdrives, if that is not enough, use any physical attack from tidus or auron. megadeath is casted once you beat 2nd form. you must alive if you in zombie status.
the third form is the hardest. first, use tidus slice & dice, if you manage to successfully hit the gray meter, then use bahamut mega-flare for overkill.
If you not, use auron overdrives to give she 4th 9999 damage. have tidus physically attack her for the fifth 9999 damage. have bahamut use megaflare to toast her to hell(overkill). your bahamut mega flare must hit about 20000 damage in order to win.
just to tell the strategy earlier for you.
hope you good luck!:)
P.S- I also make my Kimahri go through Lulu sphere grid and its rock!
