If you lose to a new character, they won't reappear immediately. Go to Games & More and look for the glowing "door" icon in the bottom right to rematch them.

Copy the contents of the downloaded save (typically system_data.bin and other files) into this folder.

: Win or lose (suiciding off the stage works), a challenger should appear. Defeat & Reset

No risk of save file corruption; feels "earned." Cons: Takes roughly 1–2 hours.