I did this achievement on the "A House Built on Sand" mission
I used Signature 47 Suit, a Lockpick and Fibre Wire.
I headed to the Rugshop where that guard has the key for the backdoor of the VIP club, lockpicked the door without anyone seeing. Headed up stairs picked up a brick, threw it near the ground and lured the bodyguard towards me, and i subdued him took his key, I also subdued the Rug Saleswoman and put both of their bodies in a nearby box.
I ended up poisoning Kong Tuo-Kwang's Hookah Pipe after distracting his guards in the toilet area and subdued them, then sneak into the area where he sits down where his guards used to be guarding where the hookah pipe is. then he becomes sick once he trys smoking the Hookah pipe. he heads to the toilet where you can kill him without a chance of anyone else finding his body.
the same can also be said, if you follow around Matthieu Mendola long enough you will noticed he stops to eat some Tea Leaves, if you time this right with poison the tea leaves pile without anyone seeing you. You can make him head to the toilet once he trys some and kill him he has the documents on him, pick them up and head for a exit, all without changing your starting suit outfit, being spotted trespassing or doing something suspicious or any bodies found, and only Kill the targets. if you done all this then the achievement should pop