Pretty much the title. Getting immortal Sister Crimson/ Brother Segarius/ Astros against elemental builds because they happen to roll elemental resistances on their MI’s is neither fun to play against nor a proper way to balance them.
Both builds have around 100% resistance reduction excluding Viper devotion, so it’s even higher with that. And no, just because both builds include Arcanist class which lacks RR, I don’t think that is a fair counter-argument. Other damage types don’t hit walls like this with even less RR.
In conclusion, I want to suggest that these bosses to simply not be able to wear armor gear, or something similar that happened to Ascended Myrmidons in Crucible ( though they are still sometimes immortal too on rare occasions).