Don't know if anyone has posted this yet, but If you're like me and already did the mission "The Cowl Does Not Make the Monk" for the "No-Hitter" achievement you might want to just go to an area with lots of guards, such as Campanile Di San Marco and use smoke bombs. You can then simply switch to double hidden blades and assassinate them when they are distracted by the smoke. You can carry three smoke bombs so you can just throw one, kill 4-5 guards and repeat.
Also, great guide. Thanks a lot.