This should come Naturally. Its for getting a certain amount of Kills. Not too sure how many though. Got it in the 2nd Level
Shout Out to CLARION85 for the Tip!
There is a Hardcore Achievement for Completing the Game on Hardcore Difficulty. (One Hit Death)
HOWEVER, you can play the game on Normal all the way to the Last Boss and then quit to the Main Menu. Turn on Hardcore. The Achievement WILL STILL pop after you beat the boss on Hardcore. MUCH easier than playing the Full Game on Hardcore💪VIDEO GUIDE: