It isn't a simple question of who is the most powerful, who is the strongest, nor is it a question of hierarchies determining who will triumph. Morgoth was the greatest of the Valar and Sauron his greatest servant, so possibly the greatest of the Maiar. But the whole of both LotR and the Silmarillion show how small, insignificant peoples in terms of 'status' can beat these 'great and powerful' high status figures, whether through bravery, cunning or with the the help of the correct tools.
It was a twist of fate that brought Bilbo to the One Ring, and thus eventually to small, insignificant people causing the downfall of Sauron. Also important is pride and arrogance; Sauron had the arrogance to encapsulate much of his power into the One Ring without considering that it could be lost and that he would be unable to retrieve it. Likewise the Witch King had the arrogance to believe that he could not be undone. Perhaps he believed the prophecy about him a little too much.
One of the weapons of those on Sauron's side is propaganda if you will, i.e. their reputations as beings of great power and invincibility; they ride on their status and do not consider that the smaller people only need to reject the notion of status in order to gain the courage to face them.
In any case, the text says it was the sword itself that was the necessary 'tool' for undoing what held the Witch King together. Perhaps some of those who opposed Sauron knew that this blade was necessary, or if they really did have such power, wouldn't they have attempted to challenge the Witch King already? It seems that Aragorn might know of a tale, judging by what he says when he finds Merry's sword.
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