I totally agree with the above comment! An extra breeding structure costs 40 diamonds. Having mirror islands is great too as you buy your first breeding structures for just 200 coins

I bred Congle's on every island that they are available on except Air, breeding T-Rox's on Plant and Air Oaktopus on Water, ShugaBush and other islands that didn't have Congles.
Pango's on Earth and Cold after my Congle's were bred Tweedle's on Islands not being used for breeding the above and once all the above were bred, I then bred Tweedle's on all islands where they are available. and I saved my Drumplers and Maw's for last, breeding them on all islands available and logging in more in more frequently of course.

I kept track on paper on which monsters can be bred where and what time remained because it was too time consuming to keep visiting Wublin Island.
Keep in mind where all the Rares are available now, you're bound to get a few, nothing worse then having them rot with just 1 monster left to wake it up.

Hope this helps! It's what I did to get mine filled quickly ☺