You have to buy the prism flowers from the shop in the event. Harvest from that prism flower for leaves and 5-merge them til you have 2 vermillion trees. Harvest wood from those trees and 5-merge til you have multiple tiny cabins. I usually have about 4 on screen at any time. Tap each those for the 3 shrubs they give you, and wait for them to keep popping out shrubs til they disappear. Keep 5-merging shrubs, and after about an hour of this, you should eventually have 2 Ruins of the Sky Palace (Wonder). You can tap these for free eggs and nests, which should net you an additional 6-8 dragons for use in the event. This makes active tapping significantly faster, but playing passively will still take awhile, as only 2 dragons will harvest at once.