The silenced pistol contains 12 shots and there are exactly 12 enemies to kill. Therefore, missing a shot means you will have to start over.
The best method for unlocking the achievement is to play the level on normal and use lock-on (middle mouse button). Proceed through the section of the level with caution and try and take out the gang members one at a time or in a controlled manner.
- The name of the achievement is an obvious reference to the Assassin's Creed series.
- This achievement is unobtainable if completed on hard, only on normal will the achievement be granted.