Mario versus Kirby
You'll first play a battle in which Mario takes on Kirby. You get to choose which character you want to fight as but Kirby is harder as a foe when you pick Mario.
Minion Fight
After the cutscene in which Metaknight's ship flies over and spreads evil energy around the stadium, you'll get to choose two players to start off (if you die the other two will act as substitutes) a fight with a horde of primids.
Boss Fight: Petey Piranha
Important: Before fighting, make sure your current character has a low %, as the % from the previous fight will be carried over. This means no recovery of % or characters lost from the last fight.
Facing off with the little guys was no big deal right? Well, the Ancient Minister will send in a brute to finish you off. Petey will lock up the two Princesses in cages, and you must free one of them to end the fight. Petey is invincible, but the cages are not.
The two cages contain Princess Zelda (on the left) and Princess Peach (on the right) - you only get to save one, so pick the cage that holds the girl you want to play as in the following stages then smash it to pieces(Be Warned though picking Peach makes you face Pit and Mario which is harder but saving Zelda means Mario and Pit will have a hard time defeating a false Zelda).
Since Petey is invincible in the fight, focus on the cage and not him (he can be hit, but he doesn't have a health bar like the cages do). As with all enemies and boss fights, smash attacks deal significantly more damage than normal chains and special attacks. The best attacks are actually the ones that hit both Petey Piranha and either one of the cages, since his bar is linked to both cages. In other words, you'll do double damage if connecting.
The giant humanoid plant has only two real attacks: swinging either cage in a sweeping motion across his body, or launching himself through the air similar to Bowser's ground pound attack. When he swings one of the cages, just jump over them or move to one side. When he launches himself, run as fast as you can away from the direction he's moving (move in the opposite direction that he is). If you and him are on an edge, just run to the center of the stage. His falling motion packs a punch, and will probably KO you if you're over 100%.
After defeating the plant Wario will steal the girl you fail to save and the bomb will make the Stadium fall to subspace.