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The Metal Bros., Metal Mario and Metal Luigi, are alternative mini-bosses of the regular random metal character fought in level 9 of Classic mode for Hard and Insane mode and, as such, are the last foes before Master Hand himself.
The battle takes place on Meta Crystal and, as stated above, is only accessible if the game mode is played on Hard or Intense difficulty (otherwise, the player has to face a random metal character). To complete the level 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 Mario and Luigi. They both have one stock each. 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 lower blast line.