Boss Battle plays when you're fighting one of the bosses at the end of each stage. The title comes from the DuckTales: Remastered album, which matches the in-game title of the stage.
This is the original version of the song. It is track 8 in the NSF file and played on the RP2A03.
The Game Boy release features the same music as the NES, just in slightly lower quality. The song is played on the DMG-CPU B.
DuckTales: Remastered (W32)
This song was rearranged to pure digital music for the remastered game.