Select the car and proper race level until you're goiing up against the boss car you want to unlock. Then beat the car at the PRO-level Boss Races. You'll find them available in your pocket garage.
Unlockable: Subaru Impreza WRX STi
To unlock the Subaru Impreza WRX STi, race against the Boss Car using an American car in a Cup-level Boss Race.
Unlockable: Nissan Skyline R34 GTR
To unlock the Nissan Skyline R34 GTR, race against the Boss Car using an American car in a Trophy-level Boss Race.
Hint: Boosted Acceleration
In drag racing if you tap the nitrous button instead of holding the button down your car will accelerate quickly, but in a manner you can find easier to control.
Printable Need for Speed Underground Rivals Cheats