By selecting the StreetPass option from the main menu, you can play StreetSmash. Enable StreetPass to collect hits from passers-by and battle against them, or play against up to 12 computer players in Practice mode. All of the StreetSmash-related challenges can be unlocked in Practice mode, except for the one for collecting 5 hits.
Select the character you want to appear on your token (it doesn't matter who you choose) and write a greeting to get started.
StreetSmash is pretty simple. Your goal is to knock all of the other tokens out of the ring. Aim your token with the circle pad and press A to attack. Hold A to charge up for a stronger attack that will send your token flying. Attacks from behind are more effective than attacks from the front. The more damage a token takes, the more red it will turn and the easier it will be to knock away.
You can guard with R and evade by moving the circle pad while holding R. After evading at the last moment, immediately press A to automatically target your foe with a powerful attack.
You'll win equipment and gold, depending on how well you performed. The prizes for beating StreetPassed tokens are better than those for winning practice matches.