15 is a little tricky to get at the start. Once you get better cars you will find you are able to get a higher multiplier much easier. The best place to get this is in one of the Driftkhana maps. Find a donut spot and just keep drifting around this. If your car has enough power you should be easily able to keep the drift going if you loose it. In the background you will also be getting massive bonuses for the donut. This strategy will also let you get 2 million points very easily.
Reach a 15x multiplier