This was an easy achievement but many others have found it buggy.
This is quite buggy so I'll take you step by step on what to do ( I did ) and hopefully it will be stress free.
First thing to do it make sure when you are on the game main menu ,( where it says options, additional content and the main story) ,
See where it says " Initiates", if it has a question mark "?" then sign in club ubisoft.com on your laptop or pc etc and link it with your ps4, just go to sign in and click the ps4 icon at the bottom, there is facebook icon aswell so you know you are on the right page and sign in using your psn. Apparently this is the big issue for players. You will notice that the ? is now away. I think you need the webpage open to do this because it randomly will go off. I played one game and it had the question mark and it never worked, I signed in and played another, came second and it worked straight !
* Side note *
I had to use Firefox to sign in to ubisoft because google chrome doesn't load it basically.
Moving on on how to acctually get the trophy !
- Select multiplayer from the main menu
- Select 'game lab' on the main screen (the first screen you see)
- Choose the one on the left hand side which is public you will see, the one of the right is private
- Now 'search session' and finish a game.
A lot of people online still can't get this so hopefully this helps you guys.