The Shiva sisters fight in Final Fantasy XIII is more to prove yourself to the sisters so you may summon them than to actually fight them. To win the battle, you must fill up the gestalt meter which appears over the sisters' heads. No one will die, one sister will actually heal you throughout the battle, but you do have a timer over your head which will give you a game over if it runs out. To begin, use Libra on the sisters to determine their weaknesses, which will make you realize that physical attacking doesn't work. So switch your paradigm to sentinel, and use steelguard over and over again. When you block her attacks with steelguard the meter will fill much faster than if you attack her. When prompted on the lower right hand side of the screen to press X for gestalt mode, do so to trigger a cutscene, and you now control the Shiva sisters.