Generally speaking, the DPS without defense is a weaker target (in terms of being able to overwhelm and defeat).

The same goes for a healer. Force them to blow big cds early or tax their MP and they can be taken down. A team without a healer is easy pickings.

Overall though, each battle is situational. If the DEF buff DPS is a deer in headlights, take em down. If the healer is slow on the draw, capitalize on it. Remember all the tips here, but play it by ear, each time.