Complete any Bow Mission, side quest, and the Achievement will pop up.
The earliest you can get it is after you finish the Prologue and interrogate the Uruk. Once you've reforged the Forge Tower in that area, the icon will appear on your map.
This mission is called Swift Mercy and requires you to kill 15 vomiting Uruks in under 30 seconds. The Bonus objective is to kill 4 Uruks while continuously in Focus Mode.