When you get a card of three star or higher rarity, begin leveling it. Work on your top card, followed by those in decreasing quality. Usually by the time you have completed upgrading your best card, you will also have other good cards to upgrade. When leveling them, keep in mind if you use the same element you will get an experience point bonus. Each island you explore will have a specific element that most monsters will have. Build your deck to use monsters that are strong against that particular element. Try having two of your monsters strong against the opposing element, and two others that are neutral. Inspect your monster cards to learn what is needed for them to evolve. Use everything else to level them up. Repeat previously completed levels to farm more cards and use them to level up your best cards.