I had a lot of trouble with them, too. With levels in the low 40's, I had Penelo spam Fira or Thundara when they got together, and Ashe and Balthier attacked whichever had the lowest HP. Everyone revived when needed. It took a few tries. Just be careful not to waste too much MP or items before the battle. Run from everything after the Tchita Crystal, and make sure your defensive equipment is the best you can have.