This achievement is unmissable.
You will get it for completing the 2nd sequence in the game.
Be aware of the 2 optional tasks that you need to fulfill in order to get 100% synchronization. 1. You need to kill 5 enemies with barrels
This can be done after you have exited the train. Go to the marker and perform a jump into the haystack. From that location you can see one hanging barrel to the right (with 2 enemies standing underneath it) and one more a little leftish with another 2 enemies. Sometimes the enemies tend to wander off. If that happens simply wait for them to come back. Throw a dagger to cut the ropes as soon as the enemies are under the barrels. Another location can be found if you walk straight ahead from the haystack. Find a position on one of the buildings to the left or right of it and throw another dagger as soon as one of the enemies is under the barrels. 2. air assassinate your final target
When you have reached the secret lab, silently kill all the enemies that cross your path and make your way up to the metal beams across the roof. From there you should be able to perform an air assassination on the final target.
A video with 100% synchronization can be found down below.
(video credit : WikiGameGuides)
Hope this helps you guys!
Any improvements? Please let me know in the comments below!