Darkwood

This missable achievement can only be unlocked in Episode 2. Search the edges and corners of the map for a quarry. When you get a little ways in, the protagonist will say something like, "This is the only way in or out of the quarry." Look for a large boulder propped up by some logs. Destroy the logs to drop the boulder. I used an axe, and I had to take two swings to get some of the logs and one swing to get the rest. The boulder will fall and block the way. You need to hurry back toward the entrance, to keep from trapping yourself. (Again, the protagonist will mention this.) If you don't have an axe, I'm sure most other ways of destroying things will do the job.

SPOILER: There are some villagers further in. They're hostile, so if you go in there, you'll have to kill them or be killed. There are some lootable boxes, dead bodies, and other items, and the villagers you kill drop loot, but it's all common or low-value stuff like rags and potatoes, so you aren't missing out on anything by simply skipping that section and dropping the boulder. You'll still get the achievement if you kill them all first and trap their dead bodies.