rare immune to damage Trial of Sehkemas

Encountered a rare immune to damage in the trial where all rares have to be killed, so it was impossible to finish the trial.
Rare (level 65) was called Paingrin: kightning resistant, proximal tangibility and evasive.
Was playing a werewolf druid (level 70) using shred (physical) and lunar assault (cold) plus supporting skills and was unable to blacken even one pixel of its health bar.