The Quest should't be any Problem, but if you wonder why you can't complate the Quest: you need to finsh the Main Quest Ladies of the Wood.
Can someone confirm the Hint by SilvrgunSuprman?
I finally got this to unlock this morning. I would say this achievement IS missable. When you have the conversation with the Baron you have to agree to go with him to the swamp; even if you don't agree the Baron will convince you to come and you kill the Fiend by the Crones hut BUT you won't get the achievement. Just keep agreeing with everything the Baron says and it should unlock.