[Lostgaeriel]

I agree that Merry and Éowyn together destroyed the Witch King. I think the Black Captain interpreted the prophecy to mean that it would take someone more powerful than a man to stop or destroy him. He knew Sauron could finish him off but obviously considered Gandalf, a powerful wizard, too weak to beat him. He didn't consider that the small (Merry) and desparate (Éowyn) might team up to do him in.