I just found this out by accident. On the challenge tower level 205, where you play as Scorpion and you have to hit jade with special moves to get free coins. Well, she kept blocking and I decided to retry the stage just to see if I could hit her with more moves. Then, I noticed the game keeps adding all the coins from before you hit retry. So, I just kept hitting her with moves, waiting until the clock ran down, then hitting retry over and over. All the coins just kept adding up. The challenge pays out good too. In about an hour I made about 70,000. Since you can't even die in the fight, this is definitely the easiest way to get coins I have found. Also, don't let the stage end until you have all the coins you want, because you can't go back and do it again.