Clerics in Aion have always been a source of both the strongest heals and consistent DPS. Healing with a cleric while second nature is largely dependent on picking the proper healing stigmas. Stigmas are what separate the DPS clerics from the healing clerics.
While there is some debate about what stigmas are the best and this is not a concrete build by any means. You can replace certain stigmas with others so long as you get the vision stigma.
Restoration Belief: This stigma takes the concept of saving grace, gives it the healing power of ripple of purification and applies it to the whole group.
To unlock this stigma you need to choose Benevolence and then any combination of Healing Servant, Ripple of Purification and Splendor of Rebirth.
Benevolence: Very simple concept you use 10% more MP but you gain additional healing boost. There is no reason to run this constantly but you should use it while roaming in groups (especially if you have a Songweaver).
Ripple Of Purification: In addition to cleansing your allies this skill is your most powerful heal, allowing you to heal your entire group for several thousand points of healing use this with blessed shield to get a quick restore to your allies health.
Summon: Healing Servant: While you could swap this skill our for Splendor of Rebirth this skill provides more healing over its duration and provides much needed consistent healing for your entire group. Use this skill as an opener when engaging in a battle.
Power Sprint: If you are in PvP and you need to either catch up to your allies or retreat back to them Power Sprint makes for a quick getaway. Using this skill properly can mean the difference between life or death.
Saving Grace: This heal is reactive meaning that it only takes affect after your ally falls below 50% HP. This is a great heal to have activated before an incoming fight as a way to get your allies health up incase you fall behind in your healing.
Noble Grace: Noble Grace makes it to where all allies in your group receive 50% more healing this coupled with Blessed Shield can take your group from 0 to 100% very quickly.
If your entire group is very low try using Noble Grace + Blessed Shield the effects do stack. If you heal for 1k HP Blessed Shield will increase that to 2k + 50% from Noble Grace which should give you 3k healing.