Golle is correct about the delay. Not sure there is a way around it. However I use it to my advantage. Lets say you have 4 or 5 nukers and the first volley doesnt kill the target, but the 2nd volley is over-kill. Here is what I do.

I have 4 shamans @ 60 and equal level NPCs take about 6 to 7 lightning bolts to kill. If I did 2 volleys I would waste mana. So I target a NPC and do the first volley, then right before the spell finishes I switch targets on the leader and start the 2nd volley. The leader will shoot the new target, but the slaves will continue the 2nd volley on the first NPC. You can keep doing this to save time and mana.

I have yet to put in the time to make macros for healing, I will give more feed back once I do.