Aspect of the Writhing Dead
Aspect of the Writhing Dead has 1,600 health.
It takes extra damage from blunt and frost; it resists slash, pierce and fire; poison does nothing to it at all.
Nothing but a player can damage it, so tames, Dvergr and summoned companions do nothing for this part of the fight.
Its hardest hit is aoe at 200 poison, out of 2 attacks.
Some of what it throws is elemental, so poison resistance is worth having.
You cannot block aoe, so give yourself room to dodge instead.
Of everything craftable, 
Frostfire Sledge, 
Thunderblood Sledge and 
Frostfire Mace put the most damage through what it resists.
Aspect of the Writhing Dead is a phantom copy of 
Bonemass that 
Kall Fimbulbringer summons during his second phase. It has 1,600 health, is weak to blunt and frost, very resistant to pierce and fire, resistant to slash and immune to poison, so the blunt weapon that beat the swamp original still applies. Its run speed of 4 sits below its walk speed of 5, meaning it closes ground more slowly when it charges. Nothing but a player can damage it, and it leaves no drops.
Weak to blunt and to frost, resistant to slash, very resistant to pierce and fire, immune to poison. A sledge is the obvious answer and a Frostfire Sledge is close to ideal, because its blunt and frost halves both land on a weakness while only the fire half is wasted. Its cloud attack cannot be blocked, so poison resistance is worth more than a shield.
With devcommands enabled, this spawns Aspect of the Writhing Dead in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Aspect_Bonemass · all console commands and IDs