One of the hardest achievement in this game. You need to win your first fight at Hard difficulty. The mechanic are kinda hard to maby understand. Lucky enough if got some help from Steam community. So here is the tip:
The game still let you save your game during the fight. My tip is to save when you hit the dude, and so on. I struggled really hard on it. But if you save everytime you hit him. At some point he is knocked out. Also if you didnt hit him. Try to reload your file via ESC button.
Hopefully it helps you boys and girls.