If you already have take all assassin's QGs and having trouble like me to get this achievement, I found this location on New York city (674,-288), there are 3 assassin's guards and 2 are those who throw smoke bombs, don't kill the 3th guard, he will give a chance for those others 2 to throw de bombs, just keep the [cn_B] button press to defend the attacks and waits until one of them throw the bomb and don't forget to press the [cn_A] button to put your mask on.
And if you don't have the Denied achievement, at this locations tere is a spawn for air attacks just look for a house right at the wagon on the same location (674,-288).