In Florence after you save Lorenzo, you are given assassination contracts. The 5th or 6th one of these (in Florence) is called 'Peacekeeper'. You have 1 minute to take out 10 brutes in the south of the map. Make sure you have smoke bombs and double hidden blades. Equip your FISTS and walk up to the first group of 5 then PUNCH one to start combat. Immediately drop a smoke bomb and equip the hidden blades. You should be able to kill 4 or 5 before needing to drop another bomb and finish the job. Because the Brutes are all close together you will score numerous double kills giving you what some might think is a tricky trophy in about 20 seconds.