gandalf and witch-king

I am a new member of this site, and I joined for one reason only: to gain peace of mind after the horrible defeat of Gandalf by the Witch-king in the EE of the Return of the King movie. Unlike some of the die hard fans of LOTR who hate the movies, I love them. But this scene struck such a devastating blow to me. I honestly didn't think that Mr. Jackson could have slipped up that bad. There is just no way in hell that the Witch-king could have defeated Gandalf quite so easily. After all, Gandalf is a Maiar spirit, and the Witch-king is just a corrupt human spirit. For hours I tried desperately to figure out how this defeat coul be:

1) The Witch-king's power grew greatly due to the increasing power of Sauron and strengthening power of darkness.
2) Gandalf was not defeated yet, and would have dealt a devastating blow to the Witch-king, had not the Rohirrim arrived just in time.

But now, I am sure that it was just a purposely overlooked flaw in the movie, in this case to reduce Gandalf's power greatly, and be put at the mercy of the Witch-king only to be saved by the Rohirrim to add great dramatic affect. This may work alright for an average movie-goer, but not an avid LOTR fan. But alas, tragically, this itself does not even work because what villain their right mind would let the chance to destroy their arch-enemy slip away? Becuase of the arrival of reinforcements? Isn't that what the entire orc army was for? So for that reason, the scene just falls apart and seems very unrealistic. The only idea I have that could salvage it, is that the Witch-king was afraid of killing Gandalf because Gandalf had yet to reveal his true powers.

Well that's all the ranting and ravin that I have for now.