This achievement is story related and you obtain this at the end of the game of Alan Wake.
As for some tips on beating this pathetically easy tornado, even on Nightmare:
1) USE THE FLARE GUN: There is nothing else that is in your arsenal that will allow you to beat this boss except the flare gun. What I did was just aim for the dead center of the Women in Black with the flare and when it explodes it does quite a significant amount of damage to her buses that are circling around her.
-NOTE: If you are a horrible shot with the flare gun and cannot hit the side of a barn, DO NOT worry, there is a box on the first cliff ledge thing with an ample amount of flares to shoot at her. (When I beat her I looked at the box and I had about 8-9 left over)
2) When jumping from ledge to ledge, LOOK TO YOUR RIGHT AND LEFT! I can't stress this enough! There are flying things that are revolving around in between the gaps that will knock Alan off and make you have to restart the whole fight.
Follow these tips and you should have NO problem beating this extremely easy final boss. But serious, it was almost easier than the final boss from Dead Space
Post comments if you have any additional suggestions or problems and I will give you more advice.