I can't say anything about SCH or AST, but this is how I heal big speedpulls in dungeons on WHM:

1.) Before the tank begins pulling, have Stoneskin applied. Do not HoT until he is done with the pull.
2.) Once the tank stops, use Eye for Eye, place Asylum, pop Divine Seal, and cast a cure or two if he takes an initial spike of damage.
3.) Once the tank's HP stabilizes, apply regen, enter Cleric Stance.
4.) Swiftcast Holy, during stun duration cast Aero III, then one or two more Holy, Assize.
5.) At this point, if the tank is still stable, you can stay in cleric and dps while regen keeps the tank up. Otherwise, leave cleric and restabilize the tank.
6.) Remember that non-CS Assize, Tetragrammaton, and Benediction are good emergency buttons if the tank gets dangerously low.

Now if a tank is overpulling to the point where Benediction and DS-buffed Cure II spam is not enough to save him, then it is the tank's fault. A good tank that knows how much to pull and utilizes his mitigation cooldowns will allow you to comfortably do the aforementioned process.