Boss Fight: Duon
After the cutscene. You'll be prompt to choose four out of the six people. After you choose. You fight Duon.
Duon is a nasty boss to fight, but really isn't very tough once you know his attacks. The blue half uses close range attacks. and the pink half uses long range attack. So when the blue half is facing you. Use ranged attacks (Fox's Blaster, Lucario's Shadow Ball, etc.). And when the pink half is facing you. Go in for a close attack. Be careful, if you stay too close to him, he'll spin around and hurt you. So it's best to attack a few times and run away. He also moves to the other side, changing the half you fight. Try to air dodge it if you can.