Here is the full list of all 29 River City Girls achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
K.O. the secret boss.
Reach 100% completion.
Reached Max Level.
Catch a dodgeball thrown by your ally.
That wasn't very nice!
Destroyed all Sabu statues.
Completed the game with a secret character.
Parry 10 enemy attacks.
Collected $5000
Was defeated in detention.
Enter the Penthouse.
Catch a boomerang.
Completed all of Godai's quests.
K.O. Sabuko.
Complete the game in Co-Op.
Completed the game on Hard.
Defeated an enemy with a fish.
K.O. Noize.
Find the keys in the junk yard.
K.O. Abobo.
Attacked Mr. Rudis.
Enter the Mall.
Obtain an item from the Pawn Shop.
Destroy a vending machine.
Defeated an enemy by hitting them with another enemy.
Equip a new outfit.
K.O. Misuzu.
K.O. Yamada.
K.O. Hibari.