Mansun

Firstly, In the book, quite a separate debate from the film altogether, I think everyone can agree that Gandalf was the second most powerful opponent after Sauron in the Lord of the Rings, as he was quoted as being "more dangerous that anything else you would ever meet, unless you were brought before the seat of the Darklord". The WK could possibly be viewed as relatively minor when compared in terms of some of the powers that dwelt in the first and second age in ME. I wouldn't put it past Glorfindel and the alike to smite him in single combat, nevermind Gandalf, even if the WK tested them to the limits. His strongest weapon really was fear, and his strength of will over others, but they were of no use against these two.

Also, referring more to the film, just because there was a prophecy that no living man would kill the WK, that does not mean nobody had the power to destroy him. It was just a comment to add hype to his character, being as Sauron would play so little a part in LOTR. We are reminded that there exists other threats from Mordor to ME than just Sauron himself, so it worked well to make the WK a little like how Sauron was shown as being mighty in the beginning of the FOTR.