Metal Bros.

The Metal Bros.  are mini-bosses fought in the 9th stage of Classic mode. They are the last foes before Master Hand and one of the three mini-boss battles, along with the Giant character and the Metal character.

Battle
The battle takes place in Metal Cavern, and is only accessible if the game is in Hard mode (otherwise, the player has to face a random metal character). To complete the trial and move onto Master Hand, the player must defeat Metal Mario and Metal Luigi. They are not unique characters, but rather permanently metallic versions of and. They both have 1 stock each and are on the same team. They are extremely aggressive and can be very hard if the player is not prepared. While they require an absurd amount of damage to be launched, they can be KO'ed rather easily by launching them into the bottom blast zone, making characters like and  extremely useful for this battle.

Since to unlock Metal Cavern the player must beat Classic mode on Hard mode or higher, this mini-boss is obligatory to unlock it.

Music
The music for this battle is called "Metal Bros. Battle", and it's a remix of the Metal Battle theme in Super Smash Bros Melee. It also plays as Metal Cavern's alternative theme.

Trivia

 * This battle is based on the Metal Bros. battle in Super Smash Bros. Melee, which functions a lot like the Metal Bros. battle in Super Smash Flash 2.