Archers with the healer perk do feel a bit too good at the moment. Especially with evasive, which lets you roll 10 times staying invulnerable for the entire duration so even if you get split up from the group you can just easily roll all the way back, as well as just making you a pain in the ass to kill in general. I'm not very well versed with being the dungeon maestro, I've only looked at the spells and such but I normally play as a hero, and archer just seems like flat out the best way of supporting a group at the moment. The only thing he can't do is throw out a quick instant reflex heal or give any invulnerability but everything else he is REALLY strong at.
As far as killing them, Imps can be alright as they don't take a hit from the front because of their shield so they can't be crit until they get either hit from the back, or lose their shield. A very large number of bees I think would also be alright, as archers do need to charge up their shot first. Forcing rolls and movement out of them should slow down the healing a bit. Even Archer healing can't stand up to multiple quick successive bee hits in harder difficulties.
Like I say I'm no experienced dungeon maestro, but it's a few things that immediately came to mind. I'm sure there are other and better ways of dealing with it out there.
However you do it, for the most part its just about overwhelming them (which is easier said than done) but it's the kind of thing that makes me think this game is designed around the top difficulty, because certain builds just make a mockery of anything you can throw at them in lower difficulties because the monsters die too fast and are too few in number to realistically threaten stuff like this for even one life let alone three.
EDIT: I guess ducks can do well too but evasive makes a complete joke out of ducks because you can just keep rolling around, completely invincible until the spin stops, so it's a bit reliant on them not having that.
