This is obviously easiest to do against a weak opponent. The first really good opportunity you have is in the prologue part of the game, after you get to the tavern. Save the game while in the tavern (so you can reload if you fail). Talking to various people will eventually trigger a cutscene. After the cutscene, when you leave the tavern you will end up in a fistfight with up to 3 fairly weak farmers/peasants. If you chose the correct option (the first, I believe, where you taunt them about how there aren't that many of them), you should only have to fight 1 of them. If you choose to use a sign to calm them, you only have to fight 2. Either way, you should easily be able to knock them out and get the achievement. If not, just reload and try again.