This is a story-related achievement and cannot be missed. It will unlock upon defeating Lewis LEGEND, the Stage 5 boss of the game. Lewis has multiple phases and you'll want to focus on slashing him up close when he's taking a breather. During his first phase he will start out on his bike shooting at you. Keep moving and when he drives at you perform the button QTE to knock him off. He will then get up and remain docile. This is your time to slash at him. Repeat until his next phase where he changes to a mech-elephant.
In his later phases, only hit him when he is standing still or else you'll take damage. He will eventually sort of break down and you can just go up to him and unleash a combo. Every time you get him into a "finish" state and perform the slice in half QTE he will transform again with new attacks. Continue to dodge and attack when he's docile to eventually finish him off for the achievement.