Minor Spoiler included!
You can do this in the first 30minutes of the game.
When you're hunting down Yennifer, you'll enter the tavern in the first village of the game. If you proceed with this quest (talk to the guys in the tavern) and (MAKE A SAVEGAME AT THIS POINT, JUST IN CASE)
then leave the building, you'll be confronted with 3 guys looking for trouble.
If you want the achievement, go with the first option
. Should be something like "only 3 of you...".
Just one guy will attack
you and you can knock him out with ease. Note, that the other 2 guys will join the fight when the first thug is knocked out. Achievement unlocks when the first guy is out!
If not, reload your savegame.
Thank you Foxy Gray for the visuals! :)