Either boost with 7 other players, or start a private game while you top the challenges.
Any 6 challenges can be: Drifting distance, Air Time, Jump Distance, Power Parking Percentage, Quantity of Barrell Rolls, Oncoming Traffic distance, Quantity of Takedowns, Quantity of Burnouts, Degree of Flatspin (in one shot), Quantity of Road Rules set by you.
If you boost, just take turns setting the minimum required to lead, top 6, and pop the achievement. Then as you take turns the following player just has to beat the lead by "1". Sometimes if you try to set these while you have less than 8 (like, while you wait for everyone to join the game), the achievement has trouble unlocking. I advise to organize all players to not drive or move their vehicles until all 8 players are in the game. Besides that, you can also simply set another "today's best" AFTER YOU HAVE 8 PLAYERS IN THE GAME, or begin a race once 8 players are in the game.
If you're not boosting, just initiate challenges to prevent other players from beating your leads (thank you DramaticSAnTA), and cancel as necessary as more players join, until 8 players are in the game.