When the Metal Box is hit by an attack, it covers the character who attacked it in metal for 11.5 seconds, turning him grey. In this form, the character is heavier, but also more resistant to knockback, to the point where certain attacks don't make him flinch if he's at low percentages. Additionally, every sound the character normally plays is replaced by metallic sounds.
In Super Smash Flash 2's Classic mode, the second to last enemy you have to face is a random enemy who is permanently metallic and has 1 stock. The battle takes place in Metal Cavern, and is the second Mini-Boss battle in Super Smash Flash 2. In Hard mode or higher, the random metal character is replaced by the Metal Bros..