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 Super Smash Flash 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.

Some information regarding to Super Smash Flash 2 is still subjected to be changed.

Master Hand
Master Hand, as he is in the Super Smash Bros. 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 has to be faced at the end of stage 5: "Pokémon Stadium" from Adventure Mode in SSF and it is one the three bosess 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 whatsover. The Pokémon faced is determinated by which playable Pokémon the player has available and is permanently replaced when a new character is unlokced. 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 permanenlty fight a Giant Mewtwo whenever the stage is replayed.

Dr. Wily
Dr. Wily is the main villain of the Mega Man 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 destoyed and Wily falls to the ground, where he starts pleading for forgiveness to the player.

Petey Piranha
Petey Piranha is a mini-boss found in the underground section Mushroom Kingdom stage from SSF2'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 Pikachu, Jigglypuff and Zero. Also the mode's beta 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.