This is in the first donjon at the start of the game. You have to progress to the second level below the one you start. Once on the second level, explore until you get to a second circular room with a rope (northwest corner) that gives you access to a new part of the first level. Go up and to the room directly to the northwest. There is a container that looks like a broken sarcophagus with the cobalt moss for Ammon inside. Take the moss and bring it back to Ammon using the door you can now open on the northeast wall. The achievement will unlock once you're done talking to her to finish the quest.
PS. For the best reward help her craft her potion and sign the letter.