Assassin's Creed Valhalla

This achievement is cumulative over all runs of this DLC.

Dokkerland is the second region of this DLC, and this achievement requires you to complete all encounters available in it. As noted above, this is cumulative, and you don't need to complete all encounters in a single run. However, it is entirely possible to do it in a single run, and even recommended, since there's no way to track which encounters you have an haven't completed yet.

Dokkerland is the most straightforward of the regions, because you don't have a split where you need to choose which direction to go. Instead, each section has two or three encounters to choose from. You'll need to complete them all eventually, to unlock this achievement, but after a few runs of getting skills and a new outfit, you should be well equipped to take care of all encounters in a single run just for ease of tracking.

One important thing to add here is that the side quest in this region, described in Freedom Fighter (40G), gets you access to the building with the hidden merchant. The enemies in this building count as an encounter towards this achievement (but there is no need to actually get to and talk to the merchant). So you'll need to complete that side quest to gain access to an encounter for this achievement.