The Shadow and the Flame achievement in AC Mirage

Once you've filled your Notoriety bar, you should hear a Horn sounded at some point as you're running around. Once this happens, a beefy enemy will be nearby and attack you on sight. My suggestion is to pick up a fire barrel from one of the areas around you (I did this at the Harbor during the Beshi mission, plenty of Fire Pots around). Once you're holding the fire pot, lock on the the guy as he's walking up to you and throw it in his face for an insta-kill and the achievement should unlock!