Select Solo Play and the game mode should be Suburbination. Select the CRRRAAAAAAAZZY difficulty with any map of your choice. For the team setup, you can pick either the zombie side or the plants side. Whichever side you pick, you need the other team to be empty. Then, activate the portal and get into the game.
All you have to do is capture all the points and just sit there. You should get 5,000-15,000 coins each time you do this trick! Hope this helps you farm some coins!