Game Mods work in local multiplayer. Turn on Perfect Manual Balance, No Bails, and Double Base Score.
Connect a second controller and start a Trick Attack game.
When the game starts, do a manual, tap
twice to do a Hand Stand, and then keep tapping
to do Handflips. Each Handflip increases your multiplier. Continue racking up points until you get to 5 million.
If you start with a grab + revert + manual combo, you'll get a big boost to your base score. Should only take a few minutes to earn enough points.