This achievement is 'Story' related, as such I'm not going to use spoiler tags in my guide, so this is your warning.
This achievement is earned after completing the quest "Ameridan's End"
To get to this quest you must first complete:
What Yet Lingers -> On Ameridan's Trail
and
Avvar Allies -> Storvacker Caged
During this quest, you can also earn
so you may want to look at that guide as well.
As there are no difficulty requirements for achievements in this DLC, I did this on Casual (or at least the boss.) But here are some tips for you.
- When you first enter the keep, head down the left hallway and there will be a Cold Resistance Tonic recipe, as well as a tonic equipping station (if you have a mage to fix the table.) On a harder difficulty you'll probably need these.
- Before the boss fight starts, you must fight 2 brutes and 4 mages. The mages tend to wait to enter the fray until the brutes are almost dead, in which case they'll probably come in and shield them. So try to fight both brutes as far from the mages as you can so they can't reach them to shield them.
- The boss pretty much works like a brute, but at certain HP percentages he will deactivate a bonfire and activate a Winter Shard near it. Other than that he doesn't really have anything special.
The achievement will unlock after you finish talking to the Inquisitor Ameridan.