i found the easiest way to get this was to start the peacekeeper assassination contract in florence. the order of the contracts is random, but it shouldn't take you too long to get it. anyway, the mission is to kill 10 guys in 1 minute. they're all in a group, they won't run away and no additional guards will join them so that takes care of the toughest parts of this achievement.
start out by slashing one with your knife to initiate combat, because if you use your hidden blade, that first guy won't count for some reason. once they focus on you, using your wrist blade is fine so toss a smoke bomb down and start knifing them. you won't be able to get all of them by the time your first smoke bomb wears off so when it does, toss another one down and go right back to it with your hidden blades.
everything should only take about 15 to 20 seconds and you'll get another achievement.