After the ending, when the "END" screen is displayed, wait until the music is over, hold Button 1 and press Run. If done right a sound will be heard. Now, hold Run and press Select to soft-reset the game. Start a Normal game and before the level starts, you'll get the debug menu.
You need a multi tap for this code to work. At the title screen, highlight "Battle Game", hold Up + I + II + Select + Run on Controller 5 and then press Run on Controller 1.