Classic Metal Sonic is a boss in Sonic Generations. Unlike Sonic, Tails, and Dr. Eggman, whose Modern selves appear in the game with their Classic selfs, Metal Sonic only appears as his Classic self, his Modern self absent from the game. The rival battle against him is in Stardust Speedway, after Chemical Plant. After defeating him, he will give you the purple Chaos Emerald.
His battle is mainly a race. You get ahead, and watch out for his attacks. After he attacks he becomes vunerable and you can strike him. After you strike him 4 times, the battle is won.