burial chamber boss mechanics

The burial chamber boss is a version of alira which has soul eater and uses the skills raise zombie, detonate dead.
I think the idea here is that you are supposed to have enough dps to kill here quickly and once she has eaten enough zombies she becomes practically unkillable.
What does soul eater actually do for her? It usually increases the other mods of a rare but as a unique she doesn't have any (listed) mods. She doesn't get any regen but she seems to get massive amounts of cast speed and some big damage mitigation?
I have a totem build, I killed her on my first try of the map but on the second one she just stopped taking any damage at some point. Definitely much more mitigation than just 75% max resistance.

Dunno about that boosting other mods thingy, but I think soul eater gives enemies 5% ele res, 5% phys damage mitigation and 5% attacks/cast speed per soul eaten. The cap for phys reduction is 90% so if you're using physical damage a fully buffed enemy can be extremely tough to kill.