When I get inside after having a talk with larienna, she teleports somewhere OUT of the dungeon and gets instakill or nearly, and I can't even save her nor find her body, could complete the quest with her dead but hrundi isn't quite happy about it
Well I searched the all
Well I searched the all dungeon of Nchurdamz, and she isn't in there. When I enter the dungeon she disappears, I've tried to bring her to other dungeons/houses, and it worked smoothly. I can't tell her to stay outside of the dungeon using "wait here" or something like that, and her corpse isn't in any of the dungeon rooms. I originally thought she was teleported to the last room and got killed by the boss, but it's not that. The last dialog I had with her was the one when she said she would follow me in (her last line in the quest's dialog)
You can install RefScope and
You can install RefScope and use it to get her (or anything really) id then use player.moveto <id>
I had the same problem. First
I had the same problem. First I was sent to help someone in Vas IIRC (mission was to kill some necromancer I think), when we entered Vas my companion I had to help disappeared and about 30 seconds later - she was killed. I had no idea what happened to her immediately after we entered because I started fighting and assumed she's with me. I later found her body far into the dungeon (IIRC she was lying just next to the next cell door, with this large elliptical structure - not Vas entry, but in Vas tower), so she had to teleport somehow there and die alone. Later I went to Nchurdamz, like OP, but I saved before speaking to the woman who wanted to kill that Daedroth. Then I spoke to her and we went inside together. She was killed almost instantly, but I did not search for her body. I just assumed it's a quest breaking bug and loaded my save. I went alone, without speaking to her, and cleared Nchurdamz alone.
So I assume going into buildings with someone in Fighter guild's quests (given in Ald-Ruhn by Percius Mercius I believe) is broken somehow. It happened to me twice - immediately after entering the interior we were separated (I was where I should be, on the other side of the door while the companion was teleported somewhere else, further into the dungeon) and the companion died shortly after (matter of seconds).
When I think right now about it - I don't know what happened in Nchurdamz, because I, knowing what happened in Vas, assumed it's a bug and just loaded a previous save and went inside alone. But in Vas, after going inside, I appeared at the other side of Vas Entry interior and I think that my companion appeared inside Vas tower instead. SHe was alone and I had to fight through Vas Entry to get there so she had to face all enemies there alone and died quickly.

Can you provide some more
Can you provide some more details? Where does she teleport to and after which dialogue?