Learning how to properly play AST

[Rongway]

You'll pretty much be full-timing Diurnal Sect. As has been noted before, you can put up a Nocturnal Aspected Benefic before a trash pull, or Nocturnal Aspected Benefic on the tank and Nocturnal Aspected Helios on the party before a boss, but whenever you do this, make sure you give yourself enough time to switch back to Diurnal before the pull or you'll be locked into it until everything dies. You don't really want to be caught stuck in Nocturnal ever; the only time to use it through a battle is in a 2xAST party. Diurnal is a better choice even when you're paired with a WHM because of MP and effective potencies.

Once you get to 60 you can do this thing...
Synastry -> Collective Unconscious -> Swiftcast -> Aspected Helios -> AoE Card Buff -> Aspected Benefic -> Time Dilation -> Celestial Opposition -> Cleric Stance -> Luminiferous Aether -> 20x Gravity
and as long as you don't have a DRK, you can stand (float?) there casting Gravity forever without healing the tank and without running out of MP.

If you're going into a duty that won't sync you (it is for your current level), prepare:
Stoneskin yourself, Spread an Arrow or Balance, Royal Road a Ewer or Spire (as of 3.1 this shouldn't fall off as long as you don't sync).
Once in the dungeon, after the barrier goes down you can Lightspeed->Protect-> 3xStoneskin on the other party members; this will mitigate any issues with in-a-rush-with-a-deathwish tanks and the party can get going and start with the killing. The party will appreciate your preparedness and effort in minimizing entry downtime.

AST has a solid ability set, and if played right you won't feel inferior to the other healer jobs. If for some reason your heals feel weak after 50, ask yourself if you're remembering to use Synastry. On top of the redirected heals, it also gives you a 20% healing potency bonus.

If you use Lightspeed as an Oh Shit button, remember that its full potential lies not in making your heals instant but rather in allowing you to perform actions in between heals without costing any time. For greatest effect, use off GCD abilities in between your lightspeed heals (ex.: Lightspeed -> Benefic II -> Essential Dignity -> Benefic II -> Luminiferous Aether -> Benefic II -> Disable -> Benefic II -> Draw). If you don't weave oGCDs into Lightspeed heals, then really you might as well have just used Swiftcast instead of Lightspeed, because the first spell is the only one saving you any time.

[416to305]

Once you get to 60 you can do this thing...
Synastry -> Collective Unconscious -> Swiftcast -> Aspected Helios -> AoE Card Buff -> Aspected Benefic -> Time Dilation -> Celestial Opposition -> Cleric Stance -> Luminiferous Aether -> 20x Gravity
and as long as you don't have a DRK, you can stand (float?) there casting Gravity forever without healing the tank and without running out of MP.

Synastry increases Collective Unconscious as well? I didn't know that, I thought it just increased actual magic spells not abilities? Would love to find out as I didn't realize that, otherwise I'd use it before CU often then.

If you use Lightspeed as an Oh Shit button, remember that its full potential lies not in making your heals instant but rather in allowing you to perform actions in between heals without costing any time.

I don't entirely agree with this. You're forgetting to mention that Lightspeed lets you cast while moving for most spells. I use it all the time, can run to one end of the arena, pop Helios, and then to the other and do it again if necessary. Great for those instances where people are spread out, especially since it reduces MP by 25%. 90% of the time I use it it's while running or moving.

[Parawill]

Synastry increases Collective Unconscious as well? I didn't know that, I thought it just increased actual magic spells not abilities? Would love to find out as I didn't realize that, otherwise I'd use it before CU often then.

No.

I would also use Luminferous Aether in tandem with Celestial Opposition to get an extra tick or two of MP, thus getting you more MP in the long-run.