A rough outline of my typical healing rotation as WHM in ex dungeons:
Put Divine Benison on the tank when they have aggro on the first group. When the second group is reached (most dungeons have two groups per segment) cast Medica II and put Regen on the tank. For particularly big pulls also use Asylum (damage levels have increased such that it's no longer available for every pull, and the cooldown reduction for lilies is too fragile to be of any real use). Cast Aero III on the enemies and proceed to Holy spam, using Thin Air if available. Pop Tetragrammaton, Assize and additional Divine Benisons as necessary to keep the tank healed. By the time I need to stop with the Holies in order to conserve MP the weaker enemies are usually dead already and the remaining ones are close enough to death that it's not worth recasting the HoTs. A Cure or two in between Stones is enough to top up the tank for the next pull. There's rarely any aggro problems.
Obviously this is with a good group, which I've been lucky enough to get in the majority of cases. If the tank is bad I'll have to use direct heals a lot more and skip the Holy spam (see my previous post a few pages ago for a relevant story). If the DPS is bad I may need to do another round of Medica II and Regen and may also have Asylum available for every pull. If the healer is bad... well, so far I haven't been blamed for that.
I'm generally able to largely ignore having one or two enemies targeting me. Medica II and Assize provide enough heals on myself that the enemies will be dead before I'm in any real danger. I'm also able to pull myself and the DPS through an ex dungeon with single pulls even if the tank does no tanking at all.