Hint - Money Laundering
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.

Hint - Orichimaru Tongue Trick
Orochimaru's special ability is to catch the opponent with his tounge, slams them down, and then uses a seal to stop chakra recovery for the rest of the round. It means your opponent can't use jitsus or substitutions for the rest of the round. It wears off if you used it in the first round and you go to round 2.