As said above (constantly), I believe Gandalf could've but would not have destroyed the Witch-King of Angmar. Gandalf is very much the engineer of the War of the Ring, making sure people are in the right places doing the right thing. He did cast down the Balrog, but that was because if he hadn't the Fellowship would've fallen. He only intervened if he had to, not just whenever. And there's the prophecy to consider. Even though a man (or Maiar Istary in this case) could've destroyed the Witch King, that's not what was bound to happen.
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"What mortal feels not awe/Nor trembles at our name,
Hearing our fate-appointed power sublime/Fixed by the eternal law.
For old our office, and our fame,"
-Aeschylus, Song of the Furies
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