For this activity you have to rack up a certain amount of points by throwing yourself into oncoming vehicles. Push or when you are about to be hit to activate a rag-doll like mode. You will often get launched up into the air so try to aim yourself to bounce off of another vehicle. Your point counter will keep going up as long as you don't touch the ground.
Try running at a vehicle head on, push to roll, and then push mid roll. This techniques launches you at the car at a faster speed and gives you more points.
If you are close to the total points that are needed, but the time is running out, you can repeatedly hit to drop to the floor and get anywhere between 300-600 points for every drop. Even if time as up, the activity doesn't end as long as you're scoring points.
You should also try driving at a vehicle head on while in a light vehicle. You will likely get ejected from the vehicle, scoring good points while also working on the Windshield Cannon challenge.