If you mean in one of the earlier games like final fantasy 5 all you really need is fire, or fira and she'll give up eventual. If you mean in 13, there's a trick. The easiest way would be first of all have Snow switch to the sentinel paradigm, target the sister that is attacking ( I believe it is Nix), second provoke in to attacking you, then for the rest of the battle have Snow perform the Steelguard ability (health shouldn't be a problem Styria performs Cura on Snow during the battle), keep an eye on the Geshtal meter, and sooner or later they should yield, once that happens for the love of god press the x button!' and they will transform into the motorcycle and that is the end of the battle.
note: the button layout is for the Xbox 360 version of Final Fantasy XIII, the playstation button layout will be different but somewhat similar.