After completing the mission "Ending It" you'll get put back into the loop.
Fastest way to unlock this is to go to Karl's Bay in the morning and kill Harriet as you have many times before, exit though the tunnels and the achievement should unlock.
To make this much easier set your game to "Single Player mode" and turn on "Infinite Reprise" and "One Shot Kills" in the options menu under "Gameplay".