my current 4lock/priest group is 33 now
i 'drive' with my priest and use the addons Grid and Clique (google them) for healing

left click = renew
right click = flash heal
middle button = shield
button 4= prayer of healing (soon to be circle of healing)

and just Click on the target that needs healing
and use my buttons mainly for warlock spells
1= wanding
2= shadowbolt
3= instant dot castsequence (Siphonlife, Corruption, Curse of agony) with a reset=combat/target switch
shift3= casttime dots (immolate, etc)
4= drain life (with a /cast [target=focustarget, nochanneling:drain life] Drain Life) this allows for button mashing
shift4= drain soul (same macro)
5= lifetap
6= instant fear (right now only on my priest, but when my locks get insta fear i will round-robin so i get 5 fears every minute)

as said, playing the priest, i mash the buttons (i got them memorized easily) while having the target selected on my priest
and clique healing does not switch target, so just monitor grid (or other healthbar mod) to heal