This achievement is easiest to unlock in Damascus. Save the citizen in front of the main gates if you have not already done so, then go to the scholars. Blend in with them. As they get past the main array of guards, start pressing repeatedly. You should be going in and out of blend mode continuously. After a short time you should get the achievement.
When you first reach Damascus after saving the citizen, you will learn how to blend with scholars. Enter this blend and exit it twenty times rapidly to unlock the achievement.