Taisen ("Great Battle")

Choosing this option will take you through the same character selection screens as the Team Battle mode, then drop you at this selection screen:

I don't actually know what this says, but regardless of what you choose it will then move you to this confirmation screen:

Choosing no here will send you back to the character selection, while choosing yes will send you to this screen while it looks for an opponent using the PocketStation's on-board infrared:

If it can't find an opponent, it will then show this screen and kick you to the main menu:

I don't have another PocketStation and know nobody who does have one so I can't test what happens when an opponent is found, but it's probably safe to assume this mode plays similarly to the regular Team Battle mode.