You have to assassinate 14 enemies in a row without dying.
It may sound difficult but very easy to achieve.
There are 18 enemies in total in that area, so you can ignore the 3-man-group chatting in the yard. All the others are easy to kill. Even if you screwed up and are discovered, you are still able to chop them down with 1-hit before they kill you.
Just remember if you screwed up and get yourself killed, you need to restart the whole chapter, not from the last save point.