I don't really understand what's damaging me in the Lightless Arbor boss fight (phase 2)

I'm now clearing this fight pretty steadily, but am still surprised that I'm struggling to avoid damage that doesn't seem very visible. This fight is much harder for me than the other T4 bosses.

I have around 14k HP (through health and mana) and 30/45/65% armor mitigation when moving/channeling/channeling focus. I take 24% less damage while channeling disintegrate, 40% less hit damage with flame ward, and 13% less DoT taken. I avoid the clear telegraphs, the rolling rock, the split beam, and the cone, and I still am frequently using focus to regain my HP (mana) during the fight. I probably die 10-25% of the time in this fight still. What gives?!