So basically Nunatak, the ice atronach boss in the Memorial District, simply does not work for this achievement.
Every single player I asked about it had the same problem.
I killed him 20+ times.
I tried multiple IC campaigns and killing it solo, duo, with a group, and even when getting the killing blow doesn't help, it simply doesn't count for the achievement.
ZoS, please, fix it, it can't take that long, can it? There are many achievement hunters out there who do not want to wait a year because of a bug.
Edited by KickMeAlready on June 29, 2020 10:31PM PC EU
Sithis Will - MagbladeDD