If you're having trouble with Seymour's Break spells, consider having Yuna and Rikku in Haste at (nearly) all times and having them NOT in your front line, and swapping them in immediatly when someone gets petrified. Quickly use Esuna/Al Bhed Potion to cure the petrification and then if possible swap them out again. That way they will never get petrified (so they never lose the Haste) but they can heal all petrification quickly. And if they do get petrified, well that's why you have two people who can remedy petrification. Just remember to recast Haste.