The Witch King's death

the witch king couldn't be killed by "physical means" but im pretty sure that he could have been killed by a wizard, or some being with magical attributes. the elves made the sword that merry used to injure him. the elves have some magical aspects about them, resulting in the fact that Merry had no magic, just a hand that could stab him. which would also lead me to believe that maybe the prophesy meant that he couldn't be killed by anything that was weilded by a mortal. the sword wasn't made by a mortal person, but the holder was, this might have been why Merry was heart.