Gandalf Vs. The Witch King

The Istari could be slain, but most likely only if they were first broken, i.e. Saruman. Then they would just become as vulnerable as the WK was when his spell was broken. But it still remains very questionable as to whether the WK had it in him to break Gandalf the White. Even the Balrog didn't manage that. So its no wonder why this thread has attracted a lot of debate, as most of us are of the opinion that the WK would have been held off to retreat from Minas Tirith in the Siege of Gondor.