This is easy, first don't forget to gear up, guns and armor/full HP, then just get to a subway, kill every one you see, police will come, kill all of them, you will get 5 star wanted fast, as soon you get the 6 star wanted go down to the tunnels and don't stop running until you are close to the edge but do not leave the circle, wait for the 5 minutes, them leave the circle and lose the wanted level.
You will be able to get the Walk Free achievement at the same time.