Solution has been edited to show updated known information
This achievement is story related but does require you to do a part on normal difficulty. I can confirm you can change the difficulty to normal once you get to this part and it will still unlock
*** Spoiler - click to reveal ***After beating the game you start next to the bed right outside the citadel at the end of the game so you can go beat him again if you did not do it on normal the first time.