What we are still experiencing as a bug is Kephess 1-shotting tanks after breath. I and our other tank would continually die instantly after the breath was cast to his Savage Arcing Slash. It wouldn't seem to matter when we taunted Kephess off each other, it was just instant death after the breath. We tried during the beginning of the cast, right before the cast finished, right after, and every time there was nothing we could do. The 4th bomber, while an annoying bug it's still able to be handled, but Kephess bugging out and 1-shotting tanks is a wipe every time.
I think you may have overlooked an important mechanic.
When Kephess casts Breath of the Masters, he also puts a debuff on the target called "Touch of the Masters". If that tank takes *any* damage before the debuff wears off (from Kephess, who has a very long-ranged cleave - called Savage Arcing Slash, oddly enough - or a tick from one of the purple circles), they are one-shot. It is not enough to taunt, Kephess has to be faced away from the tank who is dropping circles so they do not take damage from his cleave.