Situations like the one you described are ripe for Holy to shine. The stun slows damage on the tank while you're Cleric'd and buys you time to exit Cleric Stance and safely resume healing.

The big things about Holy are that it:

1) puts you in danger of proximity AoEs, so you need to know how the mob's abilities work
2) isnt cheap
3) has some threat to it

I'll often let tank get initial aggro + pull sorted out, Renew, top them off, then Cleric and Holy twice, Swiftcast, Holy a 3rd time, then hop out, Shroud of Stealth and continue healing.

It's like adding an extra DPS to your party every other pull for trivial content... all while doing your job of keeping everyone alive.

i'd recommend only 2 Holys if SoS isn't up - the aggro and mana costs become problematic past that.