I found the best way to do this during the downtown fraud (Hard) activity. Get to the highway that connects downtown and shantytown and don't ragdoll just sprint (LB) into the cars. If you get the fire bonus it will cause the cars to explode but it still counts so long as you do not touch the ground.
The highway is always full of cars in both directions and you should get some amazing height so you will have time to pick and chose cars to land on. some of the cars will slow down or bunch up for no reason so keep that in mind when picking.