Healer

All difficult bosses are easier with a healer in the party. Some cannot be defeated without a healer. All heal builds rely on Heal and Group Heal. You always max them out (and Self Heal).

Your mastery skill is Genesis. It increases the effect of your healing skills.

Thirdly, you never want to run out of mana. Your options for mana sustain are Invocation or Chakra.

For semi-active, you can either go with Sanctuary (to avoid aggro when no tank has Frontline) or Regeneration (for even more healing).

Lastly, you will want attack speed to heal faster. Song of Fire is a good choice, since it can also buff your party.

Strategy
Healers are best paired with tanks that use Frontline. As long as the healer is good enough, the tank never needs to de-activate Frontline. This means the tank will maintain aggro throughout the fight and let the damage dealers focus on dealing damage.

Healing is most effective on those who have defensive layers. For example, a shield user gets double effectiveness out of healing because they only get hit half the time. Similarly, Evasion, Weapon Block, Divination and Fervor combine well with a healer.