There's useful items down in the dirt. A ? Mushroom, a few random plants, a Fire Flower, a stop clock, and a Tannoki Suit.
There's even obstacles down there that can help you fight you opponent. A Banazi Bill, a Bullet Bill, and even, not in the dirt, Shy Guys, a Tweeter (I guess his name is), and also bullet throwing Snifts.
This arena has no custom music (theme is SMB2 Wart), no custom graphics, and no custom NPCs.
Here's a snapshot of the battle arena.
https://mega.nz/#!StdFmQgC
The decryption key... they may be doing this for copyright
Anyway, copy and paste this for key.
!N1hKF9zl_bvu6jnq-dI32NyF8R3tNWElByn8YdEsNzU
After that, choose if you wanna download with MEGAsync, import to your cloud, or download with the browser you are using.
Have fun!
This is really my first battle course uploaded.