What is the most up-to-date and easy-to-digest AST guide?

[Rennies]

Well first of all, I recommend leveling Conjurer a bit so you can get the essential Cross Class Abilities (Cleric Stance, Protect). People will argue that Stoneskin is completely essential, and it's a decent buffer for damage between pulls, but it often isn't worth the mana.

After some dungeons with Conjurer, you'll have a decent idea of how to balance healing efficiently with DPSing. Astrologian is nearly identical in playstyle to White Mage pre-30, but after that, you'll need to memorize Cards and their Royal Road effects to make the RNG synergize in your favor.

Balance is the card you always want to buff up and throw onto your team or the highest damage player.

Bole should go on the Tank so you have more time to DPS or be used to give a 50% potency buff the next card.

Arrow should be used on a Caster or Ranged DPS, or even yourself. I'm still getting mixed signals from melee players. This can be sacrificed to double the duration of your next card.

Spear is of questionable value. It's the most "versatile" card but often doesn't have an impact. You could save it for pre-boss openers for DPS if you must use it at all. Sacrifice or redraw most of the time.

Ewer and Spire are the resource cards (MP and TP regen respectively). You'll usually want to sacrifice or redraw this to get something more immediately valuable, since resources are hard to run out of unless things are going horribly wrong.

For healing, make sure you know the potencies and the cost of the spells that you're choosing for any specific instance of damage. Don't spam your highest cost spells just because they cost the most and look the prettiest, there are specific instances you'll want to use them for. For example, Benefic 2 costs more than double Benefic 1 but does less than double the potency. You should save this for when Benefic 1 alone would not save someone.