You will need Guardian XP to level up your Guardians. Fighting Warlords with a Guardian will earn your Guardian XP. You can also sacrifice unwanted Guardians to provide XP to a selected Guardian.
To increase a Guardian's Rank, you must retire other Guardians. The number of Guardians you need to retire is equal to your Guardian's current Rank. The retired Guardians must be of the same Rank as the Guardian you wish to upgrade, but can be any Level or Rarity Grade.
After their Rank is improved, your Guardian's Level is reset. You must once again bring them up to the highest available Level in order to reach the next Rank.
To upgrade a Guardian's Skill you will need to use a Skill Stone of a certain Grade (Grade I Skill Stones for Rarity Grade I Guardians, and so on).
Upgrading Skills using Skill Stones will increase the power of a Guardian's Bonuses.
While you can decide which Guardian to apply the Skill Stone to, the Guardian Skill that will be upgraded is decided at random.
Initially, each Skill is Level 0, and the maximum Level in can reach is Level 5. That means that to upgrade all of a Guardian's Skills to the maximum Level, you will need 20 Skill Stones.
Enlightening your Guardians allows them to reach their full potential. There are 5 stages of Enlightenment, and each one boosts all the Skills of your Guardian. To enlighten a Guardian, you will need to retire another one of the same Grade and Type. The retired Guardian will disappear.