So I've seen videos out there claiming this is unmissable, but I've found it can be missed.
Near the start you will enter a cave where there is a wounded creature. You need to give the creature herbs to get a key from it.
However, you need to offer the herbs while you have 3+ of them in your inventory. If you make the mistake of crafting a tonic at the campfire first, you will only have 1 herb left available. Offering only 1 herb will get you the key, but not the achievement.