It's either "The Decisive Battle" (plays when you're fighting Exdeath at the top of his castle on World 2 or when you're fighting (SPOILER)the tree form of Exdeath) or "The Last Battle" (the battle against Neo-Exdeath). You can try to find an spc version of the song, extracted from SNES, or midi arrangements of the song. If you want the soundtrack, I'm not exactly sure. It seems GameMusic has it, but the soundtrack is currently out of stock. You may also want to try out Ebay.