This method involves cheating but who cares? you have to cheat anyway for godfather criminal ranking as there is no legit method so far. So you might as well cheat for this
1) Jack a police car and park inside a safehouse garage.
2) While in the garage, board the vehicle and press R3 to start the vigilante mode.
3) Now exit the car and come out so that the garage door closes. Now input the cheat code
R2,L2,R1,L1,L2,R2,Square,Triangle,Circle,Triangle,L2,L1. This will blow up the criminals car.
4) Now go the garage and board the car again. Next level starts. Repeat steps 2 and 3 until you reach level15.
The mission is not capped at level 15 and you can go all the way up to level 100 or above. My advise is to use the same method to blow up 100 cars and cause 1,000,000 property damage as well. Both should unlock at level 20 and level 35 respectively.