In the end, you'll fight Kreia, or Darth Traya as she calls herself then. When you first get Kreia in the beginning of the game, don't level her up, at all, not once through the entire game. Because if you do, when you fight her evil form, Darth Traya, she'll have powerful One-hit KO moves, like Force Crush. Unless you never level her up, she can never use that on you.