Come on!
Come on!!
Sure he could! Hadn't a man done it? People here seem to conjecture inexistent confrontations and judge the winner by their hierarchy of "power". This doesn't exist. So
Bombadil would defeat
Saruman in battle bacause he was a higher Ainu and Saruman was the 2º most powerful
Maia? Also, this theory shows its lack of sense when one faces the "battle" between
Witch King and
Merry/
Eowyn. What could the mighty king do if he was stabbed from behind? What could
Sauron do if he could never expect that sudden strike of
Anduril (?), besides he was blind in hate to care for his defense at the moment...
What I mean is that this hierarchy can't be taken as a predictor of who's the winner. It depends 100% on the circumstances of the confront (ah! what about Saruman and
Wormtongue? A Maia slayed by a scumbag). So,
Gandalf could have defeated Sauron if they met in battle or elsewhere, just as
Gollum or
Frodo could (didn't they, after all?)