Here's my usual strategy for healing multi-pulls in 50 dungeons. I tend to run Diurnal Sect, though this is preference more than anything. Hit as many enemies with Combust II as I can before the tank's HP starts to dip. Then I toss a Combust or two, so I can switch off Cleric Stance oGCD. As the tank's HP starts to get pretty low, I'll hit them with Aspected Benefic followed by Essential Dignity, then Benefic II/ Benefic spam to keep them near full. This point is tricky, because if the tank or DPS take avoidable damage, you don't have resources to catch up until Essential Dignity comes back. Make sure to keep the tank healthy, and don't forget Synastry if you need spot healing but can't take your focus off the tank. If you're falling behind even with Aspected Benefic up and Benefic II spam, your tank may very well not be mitigating enough, but you might be able to turn the tides with Bole on the tank or Arrow on yourself.

Don't get discouraged, AST isn't as easy as WHM or SCH for healing/carrying reckless tanks. I've encountered several headstrong level 60 tanks of late seem to forget about level sync, pull hard, and neglect CD's entirely. If a tank is properly doing their job, you ought to have no problem being able to keep them up.