Again, this is all assuming that the Wizard gets to cast spells or prep.
Assume 20th level monk.
Wins initiative (not assured, but 20 dex tends to help). Maybe he has "alert" too.
At 20th level, assuming no magic items (naked combat!) we have a monk that does:
d10+5 per attack.
You get the following damage (we will assume open hand, since I have an OGL handy):
d10+5 (attack), d10+5 (extra attack), d10+5x2 (flurry).
Plus:
Either instant death (failed constitution) OR 10-100 (quivering palm).
Plus:
three stunning strikes (three con saves or be stunned)
Now, if the Monk gets initiative and hits each time (not unreasonable), then:
22+20+ 44, + death OR 55 more points of damage (99), + three con saves or stunned.
That's .... going to be rough on the Wizard.
Again, if the Wizard has time to prep, it's not an issue. But if the Monk gets to blast the Wizard right out, that's going to be a serious issue.