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What if the mage is particularly smart and uses Contingency with a Teleport the first time he is struck by an unseen, unknown assailant? A rather common defense of high level magi followed by the use of divinations to find out who struck him or her. Followed by the use of summoning spells (probably epic at level 25 +) to hunt down said monk.
Who could sneak upon on who better at 25th level, a wizard with all kinds of spells to use like Shapechange (Can we say Huge Dragon..yeah, try to grapple that monk-boy) and Teleport at his or her disposal.
Bottom line is a monk will have to catch a poorly played Wizard without prepatory spells to beat him.
Then there is also the Clone factor. "Gee, the monk killed me. That son of a pig is going to pay. In fact, that is what he will become is a pig."
Then the clever mage shows up changed. Makes good friends with the monk until he convinces him that he is a friend. During a battle he tells the monk he is going to "buff him up" for battle. DM asks, do you lower your SR and accept the spell. "Bing, Bong, Boom" you are a slug.
Lot's of options for a mage to beat a monk or survive a monk's attacks. Not as many options for a Monk to deal with a mage. Especially a monk with Vow of Poverty unless the mage is stupid enough to go toe to toe with the monk.
I don't personally play wizards poorly. I don't see them losing all that often to a monk, and even then only temporarily.