I faced the Champion Cereza Cristal’Hiss in a monolith and couldn’t kill it, I only did 3-6 damage and 12-14 when it was a crit. (see Screenshot https://imgur.com/a/iPZpgm2
I also found a reddit thread about this bug: https://www.reddit.com/r/LastEpoch/comments/1k31a8c/unkillable_enemy/
same issue, age of winter 0 corruption, doing almost no damage while it’s mentionned that it’s healing if no damage done recently … unkillable.
edit: actually I was able to kill it after killing all the other mobs on the map. It’s not very clear that we have to do that though.