Boss

A boss is an enemy, non-playable character, that represent a challenge to the player and has to be fought in order to proceed with the game or get to new levels. There are bosses in the  series as well, some got to met a certain criteria in order to be fought.

Bosses are distinctly more powerful and bigger in size than the regular playable characters. They have unique attacks and can only be defeated by depleting their stamina. There exists also minor battles with similarly powerful enemies but are to a lesser importance and defeating them does not mean the end of the game or level; this is called a mini-boss.

Master Hand
Master Hand, as he is in the  series, is the final boss from Classic mode and Adventure mode in the original Super Smash Flash and is one of the only three bosses in said game. He is a hand-like entity described as the manifestation of the creative spirit and is considered the creator of the Super Smash Bros. world, this affirmation, however, is uncertain for the Super Smash Flash series given their fan-made status. Master Hand's stamina varies per difficulty and is always fought on. Though Master Hand is big and getting hit by his incoming attacks is almost inevitable, pausing the game as he is attacking triggers a glitch that makes Master Hand's attack inoffensive.

Crazy Hand
Crazy Hand is Master Hand's left counterpart and is also the final boss from Adventure Mode in SSF, where he fights alongside Master Hand after his stamina has depleted to the half and he is one of the three bosses in said game. As the inverse of Master Hand, he is described as the manifestation of the destructive spirit. It looks not so different to Master Hand, aside being the left hand, but it is evidently more erratic and moves a bit faster when attacking. Crazy Hand's attacks are also inevitable, but the exploiting same glitch specified above, means no harm to the player.

Giant Pokémon
A giant Pokémon appears as a mini-boss which has to be faced at the end of stage 5: Pokémon Stadium from Adventure mode in SSF and it is one the three bosses and the only mini-boss in said game. It is an abnormally large Pokémon character whose attacks deal more damage, given the size of the hitbox is bigger than the regular sized Pokémon; but this also means its hurtbox is bigger, and it has no flinch resistance whatsoever. The Pokémon faced is determined by which playable Pokémon the player has available and is permanently replaced when a new character is unlocked. When there is no Pokémon character unlocked, then, the Giant Pokémon will be a Giant, as it is the only starter Pokémon character; when is unlocked, it will replaced Pikachu to become a Giant Jigglypuff; finally, when  is unlocked, Jigglypuff gets replaced and the player permanently fight a Giant Mewtwo whenever the stage is replayed.

Master Hand
Master Hand serves once again as the final boss of Classic mode in the reboot Super Smash Flash 2. Two Master Hands also appear as bosses in event #51, ?????. Coming with a new and updated custom design, his battle takes place in the new incarnation of and his stamina varies per difficulty as well. Compared to his Super Smash Flash version, he has been given many more attacks.

Mother Brain
Mother Brain is a recurring antagonist of the  series and, in addition to appearing as an Assist Trophy in SSF2, is a boss fought in event #40, The Mother Brain. The fight takes place on Tourian, a stage exclusive to this event, where Mother Brain appears on the left side. She has 200 health points of stamina and summons Rinkas, small projectiles that deal weak damage and knockback. Additionally, her eye can rapidly flash colors for a few seconds and then fire the Laser Brain Attack, a large colorful beam that targets the player and deals strong damage and knockback.

Giant character
On level 3 of Classic mode, a giant opponent is faced with the aid of two allies. As expected, it has a bigger hurtbox that makes it vulnerable to attacks but conversely its attacks deal more damage and knockback.

Metal character / Metal Bros.
On level 9 of Classic mode, the penultimate level before Master Hand, the player faces a metallic character on Meta Crystal. This makes the opponent extremely resistant to knockback, but also very heavy, making them vulnerable to spikes. Reaching this level on Normal or lower difficulty, the metallic opponent is a randomly selected character from the roster. However, reaching this level on Hard or higher difficulty pitches the player against the Metal Bros.: Metal Mario and Metal Luigi. They are, as the name indicates, metallic versions of and, respectively, and fight more aggressively than the random metal character, which inherently makes the match harder than the latter as the player has to face two opponents at once.

Dr. Wily
Dr. Wily is the main villain of the  classic series and is one of the bosses yet to be featured in Adventure mode from Super Smash Flash 2. Wily rides on his Wily Capsule from Mega Man 7, which disappears and reappears from side to side. From his capsule, he can shoot a wide variety of attacks toward the player. On higher difficulties, Wily's attacks get stronger and also gets some different moves. When he is defeated, his capsule gets destroyed and Wily falls to the ground, where he starts pleading for forgiveness to the player.

Petey Piranha
Petey Piranha is a mini-boss to be found in the underground section stage from Super Smash Flash 2 ' s Adventure mode. The way he attacks is currently unknown.

Other
Rayquaza and Cyrus were confirmed to be boss characters in the scrapped Adventure Mode: The Flash Of Shadows whom would be faced by, , and Zero. Also, the mode's final boss, Umbra, was intended to be fought. At some points players would have fought Beast Ganon, Porky, Kraid, Meta Ridley, "Dark" Dedede, a CuBot warship, Sasori & Deidara, and the CuBot Alpha as stated on the beta storyline. Since the story got rewritten and everything was scrapped, all the bosses have become unconfirmed.