Ok, so using his knowledge of the body and harmonic resonance, the monk strikes quickly twice in the same spot, the first hit causing no appreciable damage, but the second causing a rippling shockwave throughout the internal system of the dragon, for about 1D12+strength points of damage, which is enough to cause a normal human's innards to simply explode.
Or, maybe it's a shot to a tendon, nerve or other area that just really hurts, so that the dragon is distracted, and then more vulnerable for the hit that does it in.
Or, the monk's fists simply strike with the force of a cannon ball, crushing muscle beneath the bone and his fist, perhaps even splintering bone. Enough force to cause metal fatigue, and strenuous cracks in stone.
Or, perhaps the monk has a field that warps the laws of physics, causing his punches to do far more damage than they should.
Or, as a rite of passage, each monk is required to dip himsself hands in a sacred river, that, over time, magically endows them with striking power.
