Stopped with story-mode, and played the "classic" mode option next to it. Tried Wario, Link and Pikachu.
Inside were a lot of nice stuff. like:
1. Special characters appear (I think they're all NPCs). Like metal wario, giant pit.
...and like some people guessed, the boss is:

2. You get your selected character's trophy after beating him. You also get coins, and also rewards in a glass shelf.
3. Even the end-credits after the boss fight is a shooting mini-game...

4. Then you'll see this:

(Dunno if all are like this) Beat the challenger, and you got a new select character! (just got C. Falcon)
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Update: glass shelf only comes up after you've done a specific challenge (like complete destroying all targets the challenge between rounds in classic mode). Don't use the hammers found at the bottom, they are scarce and should only be used for the rewards with the harder challenges (too late for me though....)
Unlocked characters: C. Falcon, Marth, Luigi
