Yeah, nightmare's kinda a pussy. I mean ... you take away the giant eye-sword and the nipple-eye, and he's just some emo blond guy with a slab of metal. He can't even control the power of a legendary evil sword.

Guts, on the other hand, has killed beings which were allegedly immortal. Killing the unkillable is pretty far above and beyond the limits of just having a big eye-sword.