I found this very good method in an FAQ for if you're having trouble:

have Rikku on overdrive and mix Trio of 9999 (look it up if u don't know it), then use a water gem to kill her 1st form, Slice & Dice for her second and then use Attack Reels on her final form, this will kill her without her even getting an attack in.

And before you say it she WILL cast Megadeath on you but since you will all be Zombied from before it shall miss