Protect the Priestess! "It is the Torch Keeper's duty to bring Light to shatter the shadows of evil."
After gaining some map control, try to protect the Priestess so she can break the boss's shield and then take him out.
For every PvE mission, there is a campaign version and a Heroic version. When you first begin the game, you can only participate in campaign missions. Upon collecting 50 sigils, you will then unlock Heroic missions.
Campaign missions will grant rewards after the first successful clear with ANY family. Heroic missions must be completed with EACH of the 5 different families in order to receive the mission rewards.