This is best done after the game and you have gotten 100% finished with the game.
If you don't have 100% most likely you haven't raced Andrew a second time.
Once you have 100%, it's best to do this on the Ninja and fully upgrade the bike.
If you don't already know, it helps a lot on a few of the turns to hit nitro as you are finishing some sharper turns. this helps you corner better. No idea why that is, seems like it should be the opposite.
Also you have to be very careful, cause you can't get more than 25% damage.
If you have more damage, just restart the race. You can see by looking at the speedometer. There is a yellow line that shows how much is 25%.
And at the end, make sure you got all of it at the same time including the time limit, if not you must start over.