Had the same issues, these mobs completely killed my entire group after iceblocking my tank no matter what I did - no on heroic it's pointless even to think about CC since most mobs won't survive 4 critical lava bursts and 4 lightning bolts which land just after that ...

Out of frustration I stopped doing instances almost completely and just leveled to 80, swapped my group setup from paladin, priest, warlock, mage, shaman to paladin, shaman x4 and et voila - easy mode activated. :>

Xorn

by the way: have fun in utgarde keep, first boss is going to be a pain in the ass and the last one is going to kill your toons one by one (if seperated) with his throwing axe if he doesnt kill your tank before that and than slicing up your group while laughing his ass off ...

I hated it so much then, yes.