Be sure to give your Minos the best equipment. Some of it will be available to equip right away. Other equipment – usually the better and more rare equipment – will be unavailable until they reach a certain level. Focus on leveling them up until they can equip it, in that case, because that better equipment makes a huge difference.
That will include defensive equipment such as the wooden shield, and attack equipment such as the stick. Go to the islands and pick a battle, and you’ll see a list of the possible drops in that area pop up. Play until you get the drops that you want, then equip them on your Minos. The rarer the equipment is, the less likely it will be that you’ll get them from drops, but keep fighting and you will get them eventually.
Gemstone Island is a special island that is only possible to earn from a $9.99 in-app purchase. However, this is arguably the best deal if you are going to make an IAP, because not only do you earn 100 gems, you get quests for 5 free gems a day for as long as you have the game. You can even delete the game, bring it back and restore your purchases to get it again for free.
You can summon the Guardian of an island, which is the rarest form of Mino, if you have the stones to do so. You can get the stones either in mega chests on the island, which pop up VERY rarely, or you can find them inside the Dragon Chest inside the main store. That chest, of course, costs 150 gems, the most expensive out of all of the chests.
The items chests are the best deal when it comes to pure gem expenditure. They only costs 5 gems apiece, and they give you rare equipment, especially weapons. Open these if you want a quick fight fix, so that you can immediately strengthen your Minos and send them into much tougher battles than they normally would be prepared for at their level.
Fight the same battle enough times in a row and you will get the Mega Chest. You’ll see the check marks appearing on the pop up screen and the closer they get to the chest, the closer you are to being able to open it. Typically, you will have to fight the battle five or six times, then you will be able to claim it from the pop up menu before the battle.