Looks like Mario didn't appreciate the beating he got previously, so now he's back, challenging you to a one-on-one match. This time round however he's donned his Metal Cap to become Metal Mario. He's going to be very tough to defeat, with boosted attack and defense, which means you'll have to pummel him extra hard before you can send him flying, whilst trying to prevent him from doing the same thing to you, which can easily happen.

Early on, most attacks will not even phase Metal Mario. Keep attacking him with your strongest moves, and if you have any projectile attacks, try to use those too. Throwing him isn't a good strategy, as he won't really go much of a distance. Metal Mario is very heavy, so if you manage to send him flying, he doesn't stand much of a chance of getting back on stage.