This will be an extremely frustrating achievement for a lot of people, but bare with me, it is very possible under the right circumstances. First off, progress through the game to unlock the FMX bike, and the "Trick Master" track under "Cactus Challenge". Now, I highly suggest playing this skill game over and over again until you memorize at LEAST 8+ unique tricks. Now that we have a feeling for some of the tricks and the physics associated with FMX, it's time to go for the achievement. Getting this achievement is possible on a few tracks, but it is recommended to play the "Airtime" track under "Skill Showcase" event, as it will be the most manageable for most players.
Now, for a quick explanation of how the scoring in FMX works:
1 - Multiplier
2 - Unique Held Tricks
3 - Points
4 - Landing Bonus
All the listed things are extremely important factors when going for this achievement. The first objective is to build your multiplier. Keep in mind, significantly slowing down, crashing, getting a bad landing, or preforming "stale" tricks will bring down your multiplier. Holding a unique trick until the title becomes orange will help build your multiplier faster. It is important to make sure all your landings are perfect landings also.
Now, with all that out of the way, here's the best method for the track mentioned above, although you are free to do small variations yourself. From the start, pull a Superman + Another Move (Such as Driller, Slash, Kiss From Above, etc...) while doing a front flip. Once you land, continue with full speed and pop a wheelie down the ramp. Once you shoot off the next ramp, start rotating backwards, and pull an Underdog + Another Move (Such as Driller, Bow, Ruler) and stomp the landing. Use your brake to slow yourself down to have a smooth transition into the next ramp. While in the air hold a Coffin while preforming a front flip. On the next jump, hold a Proud Hero, again, with a perfect landing.
This should have your multiplier at around 6x-8x, with your bike headed towards the giant fan jump, which is the jump we'll be using for this achievement. Now, I've attained 20,000+ points using a variety of different moves, and you're free to alter any of the above steps, but I recommend doing a Going Up / Going to Hell / Going to Heaven + Polevault / Ruler while doing a backlip or two. Upon the perfect landing you should be able to score anywhere from 22,000 all the way to 36,000+ points, unlocking this achievement. If you come up short, just continue the track, and you'll have a few more opportunities for big air!