All of Maiev's troops have Stealth . In the forests of Ashenvale, there's a reason to be afraid of your shadow.
Spells, Unbounds, and Fire Elemental with Immolation Aura can unstealth the waves sent by the boss. When your push is near the boss, use an Unbound Mini to waste Maeiv's Fan of Knives ability to prevent damage to the rest of the push.
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.