SPOILERS FOR DLC 2.
This will unlock as soon as you beat the final "boss" of the DLC, which consist of
The only tricky part I found is the birds attacking from all sides, so be sure to stock up on flares to be safe.
If you manage to do everything without falling to the last bar of health, you will also unlock the Iron Will achievement:
The Iron Will achievement in Alan Wake worth 25 pointsSpecial 2: Survive the final encounter without being seriously hurt.