You will most certainly have to boost this as it is very unlikely that you will naturally be able to find or fill a full, 6-player game. See THIS thread for setting up a boosting session. If all 6 players are interested in getting this achievement, it will take about 2-3 hours total for everyone to unlock it, if there is no server issues, which you will likely run into. To finish the six games as fast as possible, all players should just keep rapidly placing their tiles in any possible location. You can tell which locations are available as the game will highlight certain empty spaces for you. The person that wants to win just needs to place a single follower on a road, city, or cloister. Every other player puts no followers down at all. This will ensure victory for the correct player and that the games are completed quickly. Also, make sure to turn off the River II expansion as it just adds more tiles that must be played to each game. Games can also be played over days with toast notifications. You only need all 6 people online at the same time to start the games. Once they are started, all 6 games can be continued until their completion simultaneously.