Know that not all tanks that pull big are really capable of the pulls they are making. If you find that your tank would be dead in 3 seconds were you not casting a constant stream of heals, the tank is probably under geared.

Throw a regen on the tank, hit med II, pop divine seal if you really need to (I usually do not) and then go to town with holy spam... Keep watch on the tanks health and slip out of cleric stance at 50% so you can resume heals, rinse repeat. Throw a SS if you need to during battle, keep your regen and med II up.