Board Thread:Questions and Answers/@comment-33055384-20170912014013/@comment-25404524-20170912040830

There are many possible ways. One way is to go breed the Rare quad monsters with their respective non-Rare versions, i.e. a Rare Entbrat with Entbrat, Rare Deedge with Deedge, and a Rare Riff with Riff. Not the best option, because it won't get anything other than those Quads, which can miss out on less common monsters like your Ethereal Monsters.

One other suggestion continual breeding with Entbrat + T-Rox, Deedge + any triple-element monster except Bowgart, and Riff + T-Rox. That way, a lot of times you could breed to get chance to get Ethereal Monsters, which give quite a lot of coins.

Actually, more suggestions include:
 * Filling up heaps of Brumps/Zynths. Might impact on production of Ethereals, but there will also be quite a lot of chances for lots of coin productions alongside food production, of which food production can be used to level up more high-coin-producing monsters such as Entbrats and Ghazts. If you produce enough food via Wublins, there will be enough food to level up monsters to Gold Island for a "free" achievement, rewarding coins.
 * Purchase Rare Entbrats, Rare Deedges, and Rare Riffs as soon as possible, while they are available. Also Rare Shellbeat and Rare Quarrister, once you unlock Water Island and Earth Island respectively. In fact, Rare Riffs produce the most coins out of the other Rare Quad monsters.
 * Breed primarily Quads and place them to your island(s). I used to do this, and I have gotten 4 Deedges on my first account's Cold Island. I have gotten lots of coins from that.

Hopefully, with enough participation in the game, you will probably get a good amount of coins for more coin-requiring stuff. Best of all, there is an Anniversary Month Celebration with every day of free gifts. Check the mailbox for more info.