Once you get the bouncer (as in the part that makes your key object bounce around), start a Scoretacular challenge in any level. Now go crazy with the bouncers.
Just make a tunnel of bouncers, starting from the gap, to the goal at the end of the track (make sure the bouncers are placed, so that the bumpercar can make a little start in crossing the gap and can start bouncing).
By tunnel, I really do mean tunnel: both above and below you should place bouncers so that the car can just keep on bouncing to the exit.
It may help to only place bouncers at the bottom for about 2/3 of the way to the goal so that the car can sort of settle down near the end of the track. Because you're garanteed to hit the multiplier that hangs across the gap, you'll see your score and multiplier go through the roof. If the car eventually does NOT go through the finish line (or exit), just move a couple of bouncers so that the bounce will be a little different and with at most a couple of tries you should get this. I got an 99 multiplier and a score of 999.999 points the second try.
If you can't get it on your level of choice, just use a different scoretastic level using the same 'tunnel' method. Just make sure to hit a multiplier on the way to the goal/exit.
Thanks to Spletchie also note that you can just add a tunnel of bouncers to a challenge you've already solved and you don't even have to reach the goal/target in order for the achievement to pop.
Good luck!
If you need help, please PM me.