The issue that my guild and I had with him was that he was casting his tremor attack way to fast. He would activate the tremor attack and we would be safe in the bubble, however soon after he would do it again and Warmaster did not have enough time to cast the bubble again. By the time the bubble was back up it was useless because the AoE was just falling through the bubble and killing us.
edit: By way to fast I mean he would cast it, then roughly 10 sec later he would cast it again.