A battle with Petey Piranha in Boss Battles Mode. Note that the cages are empty, and the sky is blue, like in The Great Maze.
The goal of Boss Battles Mode is to defeat every boss in the game with a single stock, or two characters in its co-op mode. When a player begins Boss Battles Mode, they are immediately dropped into combat against one of nine bosses (not Tabuu). Once a boss is defeated, the player is brought to the rest area. The rest area displays all defeated bosses as trophies, a screen that displays the face of next boss to be fought, and has three heart containers, or five in co-op mode, which, like in All-Star Mode, do not reappear even upon later visits to the rest area. This format continues until all bosses are defeated.
This format is mostly the same as All-Star Mode. However, the player is not allowed to continue after losing their stock, as it is a stadium game. The battles occur in random order, except for the battle with Tabuu, which always occurs last. Each boss has their own stamina bar as one would normally fight them in the Subspace Emissary, including both of the Hands. The final three bosses before Tabuu move and attack at a faster rate (most noticeable with Rayquaza and Galleom). The player is treated with a picture of one of the bosses upon completing boss battles: this picture varies depending on the difficulty level chosen. Stickers cannot be used in Boss Battles.
There are eight trophies obtained by completing Challenges and one unlockable character in Super Smash Bros. Brawl involving boss battles mode. Golden Hammers cannot be used to break them, except in PAL versions of the game.
Clearing Boss Battles with Fox or Falco will challange the player to beat Wolf, and beating him will unlock him.
Several of the bosses are slightly altered from their appearances in other modes.
Crazy Hand is a boss on his own, rather than together with Master Hand.
Master Hand and Crazy Hand have Stamina Bars like other bosses, rather than the usual HP shown at the bottom.
Galleom will be fought randomly in one of his two stages. His death animation will depend on when the player fights him. If they fight him as one of the earlier bosses, he will kneel to the ground, defeated (like his animation at the end of The Wilds I). If he is fought as one of the last bosses before Tabuu, he will fall backwards to the ground (like his animation at the end of The Ruined Hall).
After a boss is defeated, players will not lose a stock going past the blast lines, akin to the SSE.
Despite having a boss trophy appear in the background of the resting area after each victory, players do not "earn" those trophies.
The boss system works like the one in the game Kirby Super Star (and its remake Kirby Super Star Ultra), which has a subgame named The Arena in which Kirby had to fight bosses in a random order, except the final most difficult one which always came last.
The player can attack before the Announcer shouts "GO!"
Porky can attack the player after he himself dies, through the usage of his Porky-bots; causing damage which is likely to frustrate the player on higher difficulties.
Water effects from F.L.U.D.D and Water Gun and Push effects from PSI Magnet will make the colour in the boss' health bar blink like the boss is hurt, even though these moves don't inflict damage.