Decided to try this again tonight, but in my 10 attempts in the past hour, the Runeseer's shield never breaks during Ancestral Knowledge, no matter how much DPS I do to him (I'm 930 iLvl, last attempt was 1.02 mil DPS). Mind you, this is not at 50% health or below, but right from the start.
Has anyone encountered this? AFAIK, this is not what is supposed to happen, but maybe it has changed since the challenge was first available? I wanted to inquire before I submitted it as a bug.