Dragon Mania Legends

Each island has ruins on them, and you need to send your dragons into them 20 times. On the first island, there is the Mystic Cave and the Ghost Ship. You can first access the first of these, Mystic Cave, at level 10 - although you need to build it first. Different missions into the ruins will need different elemental dragons and at specific minimum levels, so you cannot just grind this quickly, you will have to wait until you have the dragons needed. Each mission has a chance to get a Chronisian Seal, of which you will need three for "Master of Chronos."