Instructions
Choose a fighter. In the first battle you can pick from Mario or Kirby and must defeat the one you did not choose with the one you did choose. This will be a common occurrence in the Subspace Emissary, as you will be tasked with picking characters in fights that occasionally pop up.
Make your way through the platforming sections, collecting items and defeating enemies as you go. In the first section you take control of Pit and can smash a series of orange and blue cubes to obtain bonus items such as stickers and trophies.
Pick a character when given the option during the platforming levels. Pit revives Mario and you can control either one as they make their way through the next area, for example.
Defeat the bosses. Your characters will sometimes face tougher enemies. You can choose from Fox or Diddy Kong to beat Rayquaza at the lake level. Fox's reflector is useful because you can bounce Rayquaza's beam attacks back at him. Dodge or repel this boss's attacks and then strike him in between.
Beat Tabuu, the lord of Subspace, to finish this mode. This boss has several strong attacks, including lasers, energy needles and a whip. Use three side dodges with correct timing to avoid his series of attacks. Strike him after avoiding his moves and you should beat Tabuu in short order.