Have you tried putting people in the back row? It reduces physical damage by half making it easier to heal up...your physical attacks will do half the damage but that just prolongs the battle.

Clouds limit break Cross Slash can stun the boss...giving you more time to do stuffs.

Do you believe in the power of Chocomog? It has the same stunning ability that cross slash does but it doesn't work as often.

That is my advice for most all bosses.