For the achievement, you need to reach Level 35 in Multi-Player (250.000 Fanbase).
How more people their are in the Match how more points you get. For a full lobby you get 1.000 if you become first, their is a -100 for every spot you finish lower or every player less in match. It don't matter anymore if the others are higher or lower ranked as you (like in GRID).
My boosting method is:
You need 4-8 players, all you do is start up 2 teams of 2-4 in gymkana pro tour and search at same time. When you get in a game together with everyone,have everyone hit the gas(go fowards a bit), then go in reverse(go backwards abit), hit gas(go forwards), than go in reverse. Do this ontell game is done, BUT NO ONE CAN GET A POINT"
After the time is up ever body will get 1000 point for 1st place,200 extra point if there driver rating is cautious,and 500 team bonus which add up to a total of 1700 point in 1.5 minutes !! This is the fastest way to level up online
Watch the video if you do not understand.