For all those who say that the nightmare is "IMPOSSIBLE U GUIZE!!!11one" listen the f*ck up. Get all your stats (including relationship!) as high as you can and make sure you have the double jump. ***To defeat the shadow you have to dodge its attacks (I found double jumping from one ledge to the other repeatedly to be an effective tactic). You have to make the boulders that fall from the sky hit him AND COLLECT THE DARK GEMS THAT APPEAR AFTERWARD TO INFLICT DAMAGE***