I would say never take 3 healers because its a waste of a slot that could be dps. All of titans damage is avoidable except stomps/jumps and ofcorse tank damage.

1 PLD
2 WHM
5 DPS

Helps to have at least 1 bard to keep rain of death on Titan and can ballad if WHM's need mp. Brd is probably the best dps on it because it can dodge stuff and still dps, and smn next best, and at least 1 melee for single target LB.