Enter the Create A Legend screen and intentionally make a wrestler with poor attributes (stats are all at zero). Defeat him repeatedly in ladder and cage matches to get three Green Coins with each win.
Play in a 16 team tournament. Pick one wrestler and play him through the tournament. Save before you fight the last wrestler. After you defeat him, you will get 1 Green Coin for winning the match and 9 Green Coins for winning the tournament. After winning the last match and collecting your 10 Green Coins, leave tournament mode without saving. Do not save in tournament mode after the last match or you will have to start again. Save in the options menu to keep your Green Coins. Go back to tournament mode and load the tournament you just saved. You should only have to fight the last wrestler again. Win the last match again and get 10 Green Coins. Repeat this as many times as desired. This also works in the 4 and 8 team tournaments. Note: It helps if you stack the tournament in your favor by choosing weaker opponents.
Find or create a wrestler that has an easily accessible submission move, such as one that can be used straight from a grapple. Then, have a friend be the referee in an 8-man tag match while you are controlling the wrestler with the submission move. Immediately when the match starts, put the move on your opponent and have your friend count it as if he gave up. You should win the match in about five seconds and add five Green Coins to your account.