A Bard and Druid duo presents many options beyond simple root rotting, including a fun favourite my Ranger buddy and I coined Cluster Charming. In short, root a clump of mobs from 3 to as many as you can manage with a bit of practice, making sure they are within melee range of each other. Twilight's pull effect is a good tool for pushing mobs into clump if they get rooted slightly out of melee.
Then charm one.
Use hide or invis to break if it looks like dying while charmed, but otherwise keep switching targets until all are low, dot them to death when they are.
As for Harbinger, definitely want to fear kite that one, which shouldn't present a problem for a 57 Bard who can even have druid snare to make things easier. But don't eat Ice Comets, mkay?
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Galenbor