Boss Stage is the final stage of Classic Mode and exclusive to just Classic Mode. On this stage you fight against Giga Bowser (or Master Hand in v.0.4). The stage itself is a darker version of Mushroom Plains, with brick walls instead of pipes.
Trivia[]
This stage doesn't have an official name.
Along with Mushroom Plains, this stage has gotten two variations over the versions.
This is the only solo-only stage that isn't a Target Smash stage.