Cannot hit priest in Apex of Oblation in Vaal temple

Hi I'm unable to hit the high priest boss (the one that floats around) in the Apex of Oblation. The mob goes underground and I can't seem to damage him. If I move to the opposite side of the room it pops back up, but I still cannot damage him. I had to skip the room because I couldn't get passed him.

Thanks

I am also experiencing this right now, and it sucks.