I was just going around trying to get Kaisan to spawn when I noticed that I was dealing close to 0 damage to Sister Crimson.
So insane that I had to record it! 
I was eventually able to kill her with a random acid weapon I had in my inventory, but it took a while. Here is what she dropped:
I think she had only the MIs equipped. The rares are from the orb.
By the way, Kaisan did spawn as you can see in the minimap, but he just stood there while I took the screenshot. 
avg rolls are 114 ele res on those items, iâm very much kinda assuming enemies donât get res penalties, so yep, that would indeed make her elemental dmg immune
Not impossible in campaign, and this very interaction is why they got reduced item spawns in Crucible etc as requested to not âendâ runs on certain builds against certain MI enemies
Still donât understand why at least this type of enemy doesnât have resistance cap. At least make it so they can only have 90 res at most.
Here are the real item stats. I also checked the video again and she was equipping the off-hand too, so its just an insignificant 151% elemental resistance plus the 5~10% that she naturally has. 
That is why not even my level 21/12 Distortion helped.
a modder told me when they did it the game crash frequency increased a lot
iirc from Protoss and stan Grim season i think they handled it in a different way giving enemies like an innate minus max res âbuffâ/passive? not sure
Canât say why Z didnât use that approach/whatever worked for grim league, seems like a decent fix for humanoids wearing lots of MIs 
Youâre playing a battlemage and so your only mastery source of at least single-element -% elemental resistance is OFF, which can only be temporary with the build that you have. So naturally youâre going to have a tough time in these type of situations. Maybe consider stuff like the Hand of Ultos devotion skill, single element -% RR devotion skills, Ignaffarâs Combustion, etc. Anything to boost your RR.