So after about 10 attempts, here's what I've got.
Add priority is Fuming > Servant > Umbral > Smoldering, UNLESS she is about to pop her shield. Then Umbral is top priority.
Smoldering Imp: as far as I can tell, it just is a slow ramp fireball / melee damaging mob. The cast increases in damage like 10% or so every cast.
Umbral Imp: Same thing as Smoldering, except they occasionally teleport around the boss. IF THESE ARE ALIVE WHEN AGATHA'S SHIELD IS UP, SHE IS COMPLETELY IMMUNE AND HER SHIELD TAKES NO DAMAGE UNTIL UMBRAL IMP IS KILLED
Imp Servant: Deal next to no damage, only channels a slowly permanent damage increase buff to Agatha. It's honestly fairly slow and can be interrupted.
Fuming Imp: These guys jihad the fuck out of you. They'll run at you and then when beside you, literally suicide bomb themselves (dealing about 500k instant damage) and leaving a permanent void zone.
Agatha herself just occasionally shoots out shadow crashes as well as a shadow bolt or something similar. Crashes are avoidable. Her shield can be bursted down really quick if you have the burst but the problem is managing the ability to take that shield down while at the same time having the dps to kill the priority adds.
Agatha also seems to have some sort of heal, I'm not sure if it's tied to the shield or what but she healed a total of 10% her max HP during my encounter. I haven't killed her because I just don't have the sheer amount of DPS needed (at 909 ilvl)