You can naturally earn a lot of Coins and Trophies by simply playing the game, but you can also earn additional coins by unlocking Guardians and sending them on Quests.

Additionally, Guardian Lily also earns double coins in the normal mode of the game and on quests.

Finally, if you wish, you can also sell your Sidekicks for Coins and Trophies (although Sidekicks only sell for Trophies at levels 10, 20, and 30).