I just made it to the Battlefields of Eternity in Act 5 with my Barbarian and Primordial Scavengers (particularly elite versions) are causing me a real headache. They aren't much of a threat in terms of damage they do, but it just takes forever to kill them. They've got rocky coverings that seems to block all damage until at some point (randomly? after enough hits?) they lose their armor and become vulnerable. At that point, for normal ones, I can usually kill them before they burrow beneath the ground and get a new set of rocky armor. However, for elite ones, sometimes I have to go through this process a half dozen times and it's just painfully, painfully slow.
Is there a trick to knocking their armor off or otherwise dealing with them more quickly?