I had a Novice Miner kill a dragon in one hit. Never did figure out how that came about; I think an elven caravan guard had managed to wound it a little (and I must say the way she stood her ground and loosed shaft after ineffectual shaft until it roasted her alive, buying time for the merchants to flee, increased my respect for the elves considerably) and the site of the battle was obscured by smoke from a torched workshop so it might have lit itself on fire.
Still, quite impressive even with a hefty element of luck.
Wow, that's some pretty good luck. There can be critical hits that can kill even megabeasts in one hit, and the chances increase as the dwarf, human, or elf gets more skilled and the megabeast (Or other beast) gets weaker, but you can still do it with someone unexperienced, like in your case with that Novice Miner.
For me, I embarked on an area with an open chasm, and I didn't think much of it until I get a notice that my miner is interrupted by an antman. I zoom to my miner, and he is literally surrounded by over 10 Antmen and Ratmen, and already has a few hurt parts on him. I immediately draft him into the military in desperate hope that he doesn't die, and he amazes me. He enters a martial trance two or three seconds into the fight, and is practically swinging his pick around like a tornado. There were about 14 monsters attacking him, and only 2 survived. Morul the Miner just got promoted.