I found this very easy to do. Its not that hard to do. Keep the hay for when the dolyaks need more health. Toxx is bit of a pain, but with enough catapults and defenses we got him down in under a minute.
In the update they added a new slot in the collections bank.. Have a look there. If you "Deposit all collectibles" It will send the Mystic coin to there aswell. I bet its in your bank