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