After struggling to get this achievement and having my boosting partner unlock it no problem and right on queue. I discovered that I was joining his match through my friends list instead of searching for games. It unlocked after I started joining via the search method.
So in summary, make sure you join a game by searching for the game that your friend or other created. If you join via invite or by joining a friend's game via your friends list it probably will not unlock. I did well over 100 before I switched over.