Well not really laundering, just economizing your money intake. In the start of the game your strongest character is Kakashi. Once you beat story mode you can unlock Guy and Zabuza. You can use one of those strong characters over and over again in one player mode and get 2000 coins with ten fast battles.

From there you can unlock Sharingan kakashi and Mizuki until you have all characters. Believe me it helps. Get first place in survival mode and time attack so you can get attack level four. That makes your attacks stronger and faster to win. Also don't use specials cause it just takes longer to beat opponents.