It can't be done in private match.
With little trick IT CAN BE DONE BY LESS THAN 12 PLAYERS but with aditional gold/48h trial accounts so total number of "players" will be 12, my friend did it that way some time ago and so did I with 3 buddies on my last boost session.
For example in 3 people session each of you need 3 aditional dummy accounts..
In minigames category game counts total amount of players which played through all rounds of minigame so for exmaple at beginning of 1st round players (always at least 2 players have to stay in current game otherwise lobby will shut down) sign out form accounts they started match with and sign in to one of their dummy one, join same game again and wait for start of next round. One of player who stayed score point, at next round next players swich to dummy's and so on.
Player who goes for achievement need to sign in (on his main account of course and he start game and uses dummies first) as last 12th player.
Be sure to count how many accounts you used, especially if randoms join game bec, they will count to.
IF you try to join and game says 'Session no longer avaiable" that means 12 accounts already played in game and now someone with highest score need to win.
For learning me about trick credit goes to Fuji Master PL.