There are two Battle Stages that are hidden in the game. Both of these stages use the third Battle Theme, a track that can only be heard through normal play in Battle Stage 4. The official method of accessing these stages is to plug in a Hudson Super Joy Card controller, turn on the turbo-fire feature, and hold X for several seconds. On the other hand, both stages can be accessed using Pro Action Replay codes.
Contributed by MehDeletingLater
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