AST is stronger than WHM before 60 because they have access to Essential Dignity and WHM only gets Tetra at 60. You have a very similar toolkit, apart from Cure III (which would have been useless in the situation you described), Benediction and Presence of Mind (two cooldowns that you won't be able to keep on for the entire dungeon). Healing should be exactly the same thing, but faster in Diurnal. What you're probably missing is the Holy stun and the SCH fairy that keep the tank up while you DPS; as an AST, you won't have that, but you have a very powerful off-GCD heal that will scale with his HP: the strategy is, DPS until you see the tank getting low to around 40% of his HP and then hit Essential Dignity + Aspected Benefic; that should bring them closer to full HP. If the tank is getting too low too fast, you won't be able to DPS as AST until you hit 60 and get a stun of your own.