Kill a guard ringing a bell. [Bronze / 5G]

This can be done wherever a warehouse is. Warehouses are at farms and have a restricted area around them (shown with red color on world map). They will be marked on your map if you synchronize viewpoints. One can be found in Nassau, at coordinates: 633, 784.
Once you enter the restricted area you should see the alarm bell on your HUD. Get noticed by an enemy and wait for him to ring to the bell. Quickly kill him with a melee attack when he starts ringing the bell to unlock this trophy / achievement.
You can also do this at warehouses that have already been looted. The enemies will respawn after a while.