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The first limited event dragon has arrived; the Giddle dragon; and it can be bred with any combination that includes the right 3 elements.
Plant + Earth + Light
It has a 3% chance of being bred, and the 2 dragons with the shortest breed times, that contain all 3 elements are Forest + Sand.
No fails longer than 14 hours, and no epics in the mix, so the average fail time is 9.47 hours. 3% chance translates to 23 attempts to have a 50/50 shot of breeding one,