Haven't watched anthying Shaper related, currently feel like I want to keep the mystery alive. That'll likely change in about a week or two.
But 13k... that sucks, I'm hoping to hit about 9-10k ES only. And the only phys mitigation I'll have in boss fights is fortify. But surely that attack is avoidable?
It looks like the attack is avoidable unless using something like multistrike/spell echo which will lock you in place, he was just accidentally hit, if I remember correctly. Don't want to spoil the fight, but I would suggest watching the video.
My guess that it is phys damage might be wrong but my conclusion is:
He is CI, so immune to chaos damage
Likely resist capped, so what ever damage dealt as elemental must be astonishingly huge, or it was a crit roll.
Only phys remains which is difficult to mitigate as an ES based character.
I am tempted to go ES melee on one of my characters, but still trying to figure out which will be the best class. At this stage it might be duelist due to 50% less reflect damage, or it will be shadow trickster to get a free cyclone and maybe crit to work around the damage issue. I might be able to suck out at least 12K ES with my gear in standard to test out this theory.