Getting the criminals at level 12 to spawn on the Airport runway is a good idea, every level after 12 will be in the exact same location, and if you can get it at the airport, there is hardly any chance of you having to chase the third car, and if you are using the Hunter, you can just float there and continously fire rockets at the criminals as you get higher in level. A sustainable rate is 200 or even 300 levels an hour if you do it quickly.