After having browsed the forums already and seeing that it is an acknowledged topic, I’m once more bringing to attention the invincible Nurgloth bug happening during the event. Plenty of days left under the sun, but time is kinda ticking now, and I have been unable to get the achievement for a while now because of it. Hoping for a fix soon so that I’m able to actually do it. Tried on difficulties Recruit to Champion, no dice.
EDIT: So, yeah, tried today and it worked. Maybe there was a patch or something or I just got lucky. Can’t say I changed anything. Ran it on Champion, bobs your uncle. Good luck folks!