Here is a link from Central with a few tips on it. One is from a fellow witchdoctor. Hope it helps. My big way of countering their rank 2 Vengeance Strike was to have everyone attack one harpy at a time. After the first or second attack by me or my companions they are out of vengeance strike hits and the others can pummel them with out taking on the extra damage. As they have rank 2 Hold The Line let them come to you. Just be patient and let them come to you and then attack them. If you try to go for a target another harpy may stop you and cause you to trigger attacks and you will take on extra damage from one you were not focused on. So let them come to you ( for your melee fighters that is ) and then attack one at a time. You can pummel them all you want with your long range witchdoctor powers. Make use of your charm and let one use up all that vengeance on one of their own. Also, throw out a lot of minions to distract them while your melee fighters take them out, one by one. That's how my witchdoctor did it.