I was confused by Pedegree's explanation, so here it is simple, if you don't want to watch a video.
1. Go to Sandbox, and choose to edit a level
2. Clear the level, so nothing is on it, and it will be unlimited with no end.
3. Play the level
4. Alternate between doing Wheelies, and doing Endos, as quick as it will let you. (Move back and forth on the left analog stick)
5. Once you get over x100, press start and click the save and restart from the start line (Thanks to thatkidfromohio)
6. Achievement Unlocked
Alternate Method:
5. Once you get over x100, press start and exit to menu
6. Press B so you are on the Sandbox level selector screen
7. Select the level you just played, and select upload
8. Upload the level to anyone, even if they don't have the game.
9. Achievement Unlocked.