There are two methods to choose cards. The first is to create a custom loadout with your best cards. This allows you to always start the game with them. The other option is to randomly get three cards assigned. However, you can reshuffle the cards. Do this at least once to increase the odds of getting better cards. Tap the cards you wish to keep and only the others will be reshuffled. Whenever possible, fuse your cards. You will usually play more frequently with random cards instead of the default ones selected for the custom loadout. The better those cards, the more chances you will have to complete your mission. Save the red rubies obtained from treasure chests, completing a level, or playing the special mines on card packs. When you have 40 rubies, purchase the VIP Bundle from the shop. You will get five free cards and at least five guaranteed Legendary cards.