I suggest for the OP to take Diurnal stance. It's more efficient in MP cost than Noct, especially the Aspected Helios. If your party constantly gets heavy damage then it's not AST's fault, other healers will struggle as well with that kind of party. There's always Lightspeed+Helios+Helios+Helios if somehow needed but personally, up until now, I've been running ex dr as AST and I've never used Lightspeed in that fashion.

As for MP management, well, one thing that I noticed that if you spam Benefic II, you'll run out of MP in no time. So, for me, I'll just use AB, AH (in Diurnal stance) and Benefic I, occasionally Benefic II, If Enhanced Benefic procs, I can put a burst healing for up to 11K HP (Benefic I+Benefic II). About LA, like everyone said, you should use it with The Spear if possible, if not use it when your MP is at 70%, 50% if CO is still up. Also, if you draw The Ewer, don't burn it, use it on yourself. It helps a lot!