It's tougher then the description would lead you to believe. You need to clear easy mode with all 8 vehicles. Each one has one level against two opponents, then two against three opponents and finally a "boss" battle. I suggest starting with Torque and then working your way to the right as this seems to be the right order for easy to use (Torque) to hard (Molo and Beezwax).
If you die on a level, you can just restart it as many times as needed until you beat it.