I heal ZA and ZG all the time as disc (not AA) and have never had a problem keeping everyone alive. (356 ilvl)
For tank healing I just keep Penance, PoM, and PW: S on cd and use Gheal if the tank drops below 50%. When everyone is taking damage I just try to shield the people taking the most dmg (usually melee) and throw in 1 or 2 PoH.