For me there was no need to replay any missions. I had completed the game already and set my marker for the Blacksmith in the Constantine District - just north of the Romani HQ.
He can be seen stumbling around near a boarded up Well, and a red crosshair will appear over his head. Simply beat him up with your fists and the achievement is yours.
Don't worry if you've previously come across him and unknowingly killed him, he will definitely re-spawn as I had assassinated him once or twice already.