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human
[LOTR] How was the Witch-king of Angmar such a problem for Gandalf?

Gandalf is often mention as the only being possibly capable of taming the ring, after Saruman he is the most powerful Maiar send to Middle Earth and following his resurrection after his fight with the Barlog arguably the most powerful.

From a celestial-being point of view he was of the same category as Sauron yet the King of the Nazguls appears to be to much for him? Being able to crush his stuff symbolising a bigger potential in magic and willpower? 

I know that the Istari are supposed to just guide the people of Middle Earth in there fight against evil and so they never just waltz in to Mordor magic and swords blazing but I would imagine that while being directly attacked Gandalf would have the right to use some of his true strength especially if he already directly used magic in the defence of Gondor.
gpt
I do believe it is mentioned in the Silmarillion that Gandalf and the other wizards were not permitted to use their full maiar abilities to save Middle Earth. They were meant to rally the forces of good against Sauron. So Gandalf couldn't just go full maiar on the Witch King. 

Edit: Sauron is not a fish.