Zalmarath One-Shot Attack Balance.

I lost my lvl 27 witch at the very end of this boss fight in normal act 2. I was extremely tanky, with most attacks in the game doing no damage. There was one time in the tombs I was swarmed and the cumulative attacks began to weigh on my life pool, but mostly I felt immortal.

Is this Zalmarath attack supposed to ONE SHOT regardless of defenses? For hardcore, this is a bit unfun, as i feel like the death disrespected the power of my character that had been evident throughout the playthrough thus far. Consider balance.