Immortal Visceris

I was fighting Visceris in Overgrown Ruin and he turned immortal and stopped taking damage.

I am using an ice nova trapper. After taking Visceris to 75% health, I stacked traps on the spot his first add (the spider) appears. These traps continued to trigger on Visceris, keeping him in that spot frozen while the spider appeared and died to the traps. At this point, Visceris was still at 75% health but with energy shield - once I reduced his energy shield to 0 he moved to spawn the second add, which I killed while Visceris was perma in ice nova trap damage again. After killing the second add, Visceris began moving and attacking as normal but I could not drop his health below 75% where it was since the first add. His energy shield regens and can be damaged like normal, but his health won't budge.

Edit:
Forgot to mention that the map was a shaper stronghold at the time, if that matters.

Edit2:
And to clarify, I was keeping him permafrozen with ice nova such that he couldn't disappear into is ground portal after summing the first and second add.

Same thing happened here. Visceris with Ice Nova and Frostbolt. Think I perma froze him and broke him.