When choosing cards, consider their base hit points, damage, and skills. However, a low HP and AP card with very good special skills may be useful. Always have a healer in your deck. Cards that have special skills that can be activated faster are always useful. Also consider cards that cause multiple damage or freeze the other cards. Look at the time required for each card to attack, indicated by the number in the bottom right corner. Try to use cards that can enter into battle sooner, even though the slower cards usually cause more damage.
Cards
By adding friends, both of you can take cards from one another into battle for easier wins. Try to add people that have top tier cards or at least a two-star card. You are able to use a friend's card in battle every ten minutes. Collect as many cards as possible. There is no limit to the number of cards you can collect. You can then decide which of them to level up. One star cards should be sacrificed to boost other cards. Try to increase out the hit points and attack points with the remaining cards. It is better to have many average cards than a single superior card. As you make progress and collect three-star cards, prioritize them for enhancement. Update your cards every five battles as you get new cards. Sacrifice them if they become outclassed by more powerful cards.
SP
If you have SP available, use it to explore dungeons. You will have a chance to find cards and materials that are required to upgrade. Replay each level of a dungeon until you unlock all four rewards.