You can (and should for optimal play) use multiple dresses.

Have at least 2 of the girls train in songstress's Singing Abilities, Have them Protected at the start of the battle (By your WHM(s)) and Sing away for super tough/end-game battles... You will be obliterating all of it ^^~

YouTube - Yet Another Trema Battle (Part 1/2)

That gives a bad example of it (Don't bother watching past the first idea of the damage aftermath... It just repeats), There are much easier ways to give full mighty guard to a party >_>;