I just completed this, I had zero troubles with healing the NPCs and switching the Cleric Stance for DPS.

All you do is just watch the HP bars above peoples' heads. I keep an eye on both PT list and HP bars on the tank while I'm DPSing, just practice more OP, you do not need to use Medica on the NPCs as neither of them take massive amounts of damage(that would be serious overhealing if you used Medica for this).