I found this easiest achieved during Memory Sequence 03, Memory 05 (Sugarcane And Its Yields). After looting the Warehouse in the mission - you are literally placed right next to a guard (unsuspecting) standing next to a bell. Walk in front of him so he sees you. Once he rings the bell once - kill him. You MUST use Assassinate (X). I tried this several times with no avail the first 3 tries. Shooting the guard while ringing his bell does not work - and if you have to hit him multiple times - it also does not work. You must Assassinate (X) the target WHILE he is still ringing the bell. Good luck.