There`s a bug involved in the sense that the cave-in ability is not always showing a castbar... but it`s pretty obvious to spot anyway as he stands still and you`ll see the cave shaking. I was keeping forcewave for those casts, so i could interrupt freely with mindsnap. If you go with Theran for extra interrupts the fight becomes pretty easy... just don`t miss an interrupt on the big one.

Make sure you upgrade gear on your companions as well. If boss-fights give you much trouble, use some presence stims... they boost your companions a lot. If you`re DPS Theran is probably the best companion to have out. As a healer Qyzen works fine. Personally I switched to full healer spec at level 30 and used Qyzen to tank. I`ve had no problem with any regular quests, and with proper use of cc and if needed turning of his aeo-attacks could solo some H2+ quests. I only replaced Qyzen at lvl 41 with a new tanking companion. level 47 now, still healer.