The Metal Bros. appear in Melee's Adventure Mode as stage bosses of Battlefield.
Metal Mario
Metal Mario more or less reprises his role as a special opponent from the single player game from the first Super Smash Bros.. You fight him after you defeat the fifteen Fighting Wire Frames. Metal Mario is not a unique opponent; he is actually a Mario that is permanently under the effect of the Metal Box item. He is agressive, and his recovery is longer, except he falls much faster. If the player has unlocked Luigi, Metal Mario teams up with Metal Luigi for that same battle, making it more difficult.
Trophy Information
Metal Mario also appears as a trophy which can be acquired randomly through the 1-P mode or the Trophy Lottery
Metal Mario
"Mario was in for a surprise the first time he hit a Metal Box in Super Mario 64. The metal cap that popped out transformed Mario into living metal, giving him great stamina, a heavy tread, and the ability to walk underwater. In Super Smash Bros., Metal Mario showed up as an incredibly stubborn midlevel boss." (Super Mario 64, 9/96)
Metal Luigi
Metal Luigi appears in Super Smash Bros. Melee with Metal Mario after defeating the fifteen Fighting Wire Frames in Adventure Mode. He only appears if the player has already unlocked Luigi. Stat-wise, he is like Metal Mario - very heavy and fast falling - because he is Luigi, he has different moves than Mario. He also appears with other Metal characters in Classic Mode in Brawl and Melee.