The achievement you unlock for beating Carver.
Loose Cannon is the 8th League between Double Damage and Showdown, where you go against a reckless man named Carver, and you need to unlock 50 lights to have the league. His car he rides is a Dodge Viper GTSR.
Carver's car.
His car is pretty fast, so it is kind of tricky to deal with. The mod it gives you is the Nitro Rift. To finish his demands, you need to shield through 3 mines, win any race without wrecking or repairing, fully Bolt 8 cars, and win Event 6 in Harumi's car.