Obtain all Lightning Cards in normal mode. Then while in battle mode, press Z at the player selection screen to switch Bomberman to MAX.
Win any match in battle mode then replay the same match without changing any other options (except for the stage, if desired). You will play as Golden Bomber during the replay.
Successfully complete normal mode once to unlock the "Group A/B" option at the stage selection screen in battle mode. Press Up or Down to toggle between Group A and Group B. Group A gives you access to the basic power-ups during standard battle mode matches, while Group B gives you access to the more advanced power-ups.
Successfully complete level 4-3 to unlock the "Mini Game" option at the main menu.
This trick works for any battle game mode. Immediately before the battle game starts, when "Ready" appears, use the C-stick to change the angle of the field. You cannot do this after the game starts.