Buy the lowest level motorcycle (the Ducati Monster), and max out its stats. The lowest bike is easy to control because it isn't incredibly fast, but it is nimble enough to avoid traffic on straightaways. In the customization menu in the garage, make sure that you choose the ability called "Zone" which slows down time during those crucial moments where you are about to collide with a car.
Go onto a freeway and challenge as many cars as you can. If you run out of boost, just challenge a car and then pause the game and choose to exit the race. it will ask if you want to repair your vehicle and if you choose yes it will repair the bike and refill your boost. from there just beat ten people on the freeway and you will be all set.