This achievement can be easily unlocked at Gang Headquarters in New York, which are marked as red tower icons on your map. The leader is a Master Assassin, and is equipped with smoke grenades. When you identify him (enter Eagle Vision by clicking
), begin running at him and he will begin running away. While he is running, he will occasionally drop smoke grenades. As you run up to the bomb, a prompt will appear on-screen, telling you to press
. This covers Shay's face with a gas mask. Repeat this process until he has dropped 20 smoke bombs and the achievement is yours.
noted the following, which will likely make it easier for you to unlock the achievement:
I have confirmed that you can use the Assassin Killer outfit unlocked through Uplay to obtain this. The mask is always on so you can run through the bombs without having to press A.
Chances are that you will be attacked while chasing the leader. If so, it is in your best interests to deal with the enemies, and allow the leader to hide. It is relatively simple to repeat the process and find him again. You will also likely be attacked by Stalkers, which will help gain progress towards the following achievements.
You can use the same tactic against Kesegowaase in Sequence 4, Memory 3. He will constantly drop smoke bombs as you chase him around the fort walls, allowing you to use the gas mask to counter them. However, don't get too close or he will enter combat. You can reload the checkpoint if you need as well.Ruckus360
also added this about using the Kesegowaase method:
When you fight him, he almost always drops a smoke bomb. What you can do is make your way over to it, press A to put on the mask, and then reload from the last checkpoint. Repeat until you get the achievement.
The video below shows how you can unlock this achievement and the two listed above.