Bikkuri Nekketsu Shin Kiroku! Dokodemo Kin Medal [Model DMG-NQJ]

Secret team:
At the Team selection screen, choose any team, hold Select then press A or Start. The secret 4th team (Oklahoma) will be selected.
Source: urawaza.in

Tournament skip:
During the game, if you loose one of the first two rounds of any tournament-type competitions, the game for the 1st place will be played by CPU controlled characters. In this case you can skip this unnecessary round, just pause the game, and press Down + A + Select.

Unused obj viewer:
During the game, pause the emulation and change $C000 and $C001 RAM addresses to $A and $0 correspondingly. This will switch the game in simple debug viewer for currently selected sprites list. This mode does not load an actual CHR data, so you can see only the sprites, loaded by the game previously. So if you enable it during one of the competitions, you may see the complete set of its sprites.