Below is a list of all 2 bosses that appear in Super Smash Bros. Crusade.
Major Appearance: Super Smash Bros.
Health: Percentage-based smash
Metal Mario is, as his name implies, a metallic version of Mario. A stubborn and lonely individual, Metal Mario resides in the Metal Cavern, awaiting anybody that enters his turf. When playing on the Metal Cavern stage, there is a small chance Metal Mario will come out and take on players. Unlike most metal characters, Metal Mario has a higher resistance level than normal, about 1.25 times more than a regular metal character. Based on how much damage he takes from the other players, he may choose to team up with one of the fighters for a short period of time. Knocking him out counts as a point in the match.
Metal Mario also appears in Classic Mode, as the opponent of the 4th battle. The stage is always set to Metal Cavern, and Metal Mario will be the only opponent you face in Painless, Easy, Regular, and Advanced difficulties, as well as in Hard and Intense difficulty if the total timer is over 3 and a half minutes. Metal Mario retains his enhanced resistance, which can prove to be a problem since there is only a limited amount of time and stocks.
Metal Mario features many of the same moves as Mario, but are slightly slower in execution, as well as not using any of Mario's special moves. Metal Mario never runs after his opponent, preferring to slowly walk to his target and shrug off any attacks coming towards him as he makes his way to the target.
Major Appearance: Super Smash Bros. Melee
Health: Percentage-based smash
Metal Luigi is a metallic version of Luigi; just as Metal Mario is a metallic version of Mario. Metal Luigi is very unlike his flesh counterpart; he is extremely competitive and will take on anything that could prove to be more powerful than him. Because of this, his method of encounter reflects his aggresive behavior.
Metal Luigi is not found in actual Smash matches, and is instead exclusive to Classic Mode. In order to battle him, one must reach the 4th battle of the mode, which is a fight against Metal Mario in the Metal Cavern. The difficulty must be set to either Hard or Intense difficulty, and the total amount of time spent in the previous battles must be less than 3 and a half minutes. If these conditions are met, Metal Luigi will be in the battle alongside his metal brother.
Metal Luigi uses many of the same regular attacks as Luigi, but there are no special moves. Unlike Metal Mario, Metal Luigi's moves are not slower in execution. He features the same enhanced resistance boost of 1.25x as Metal Mario, and prefers to aggresively chase his foe and combo them with grab moves, often throwing and trapping them in between him and Metal Mario.
Metal Luigi is one of the few bosses not to be playable in Boss Smash. This is likely because Metal Mario is already playable and the two would be deemed too similar, or because Metal Luigi's combo-focused playstyle would not work well in a mode that does not feature quick-paced action or grabbing commands.