This has been a problem for me. Archaon and Morathi with the new command abilities and the ability to stack saves to ignore rend have become pretty unkillable. I have been trying to focus them down with mortal wounds but their abilities, specifically Archaons ward saves make that pretty difficult. gotrek hasnt been working out either. hes just too slow, or they tie him up with horrors. Its turning into a pickle for me.