Boon Strip and Condi Cleanse are the harbinger's enemy. Untamed boon strip with sigils of cleansing go a long way. I've had success with marauder gear and runes of durability + lb/hammer against harbinger. They will try burst you so your pet will strip boons and provide that bubble. They will get immobilized cc'd by your hammer and will try to retreat. Thats when you pew pew them down with your unleashed ambush LB and skill 2 as they run away. Good luck.