Apropos of nothing much at all really, here are some examples of real-world D&D-style monk/mystics and why having them do huge amounts of damage with unarmed fists isn't really so fantastical.
Bear in mind Higaonna Morio, the guy in the first video, is over 70 years old. Also bear in mind that the guy in the second video is presumably wearing armoured gloves of some description. You get the drift.
This post is really an extension of this old one. Human beings (let's face it, usually the male ones) are really good at figuring out ways of efficiently killing each other. In a fantasy world we'd probably be really good at figuring out ways to efficiently kill non-humans too. Using tai chi on a dragon might be a stretch... But we'd probably come up with something.