Like many nameds in this zone, he regularly gets a blessing that makes him invulnerable, reflects dmg and places a dot on attackers when it's up. Players get a screen message saying "The power of malice blesses this being" when it starts and "The blessing subsides" when it's over. Stop attacking accordingly. He also has a one-shot nuke, called Carrion Plague, that does about 5M poison dmg on a single target. Single-target blockers and death saves will work, otherwise prepare to revive the tank.