you acctually can't win, but you must resist/damage her for an certain time... in case of shock, it's helpless, it really can kill with one hit, so, you must use phoenix down. After dealing enough damage on her she will use Stock Break dropping everyone HP to 1, the battle and disk 1 is over! ^^

mmm, i've defeated her with 1 hit: Limit Glove, wich deals 9999 damage if Quina's HP is at 1 (to get this use phoenix down, if you're lucky, Quina will revive with 1 HP)

ps: you can get limit glove by eating Mandragora on the forests near Chocobo Forest!