The place where I made it is Haervik Shipyard (Middle off Rygjafylke) here are many enemies standing alone.
It is not possible to complete this achievement with the bow. You need the hidden blade for this. The Chain Assassination Skill is also useful to use for this achievement.
The hidden blade you get during the quest "The Prodigal Prince"
In between the streak you should also not be seen by a wolf as you will go into a conflict.