Farm uncommon and rare Guardians from missions. Use a mission that you can easily complete or have memorized to grind lots of additional cards. You can load your team with at least ten uncommon or rarer cards before you start combining them. During battle you will find common cards as well as uncommon ones that are useless. Combine them with other cards. As you combine two cards, they will transform into a different Guardian with a higher rarity level. Card combination is based on luck and a chance of failure. However, because you are using common and uncommon cards there is little risk for possible good rewards.