On stage 7 dancing in the street level 2/10 this can be done quite easily with blowing up one car. There are 5 lanes, when the game starts go to the far right lane. The first car is black and the second is a red TNT car with a bunch of red cars in the other lanes going diagonally backward. If you blow up the red car in the far right lane it should start a chain reaction of a x21 combo. I paused and redid the level to get info for a solution and got the same results of a x21 combo. I would imagine if you blow up any of the red cars first you will probably get the same results. But the far right lane is the closest red TNT car to get to first before blowing up any other cars. Press "A" to turn the road square green, wait till the red car drives over it and press "A" again. Again don't blow up any cars first, the red car in the far right lane is your first target.