You do not need to hold off lb until the last fight and can use on first boss or as a caster a large group of enemies. The lb gauge has plenty of time to refill by the time you get to the final boss. Also when all bars are full i usually know the timing of it so a boss will be done with the use of lb, so in others words i do not use right away. Also if group composition is mellee/caster mix then normally is best for mellee to use the lb.