As far as I'm aware, it should be something like Apply DoT's to one target (Preferably one that will live the longest) > Mage's Ballad > Quick Nock + Rain of Death spam.

I mean, if we do some rather simple math (I.e. Not taking into account extra MB procs) then Quick Nock is doing 110 potency x the number of enemies per cast.

So if we say there are 6 enemies, that means each GCD spent on Quick Nock is doing 660 potency.

If lasting its full duration each DoT will only do 120 + 450 = 570 potency.

Which means that per GCD, Quick Nock is outperforming the ideal situation (Where a target lives for 30 seconds to take the full effect of the DoT)

Of course, depending on crit chance, it might be worth trying to DoT up a second target for more MB procs, though it gets a little harder to math that out...

But the overall theme seems to suggest: If the targets aren't expected to live for 30+ seconds, don't bother with multi-DoTs and just get the one set up to get some Song procs in. Even more so if you don't have Mage's Ballad ready because that's the only song that provides significant AoE damage increase.