Aspect of the Emerald Flame
Aspect of the Emerald Flame has 1,700 health.
It resists pierce; fire and spirit do nothing to it at all. Blunt and slash land at full damage, so that is what to bring.
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 bite at 180 pierce, out of 7 attacks.
Some of what it throws is elemental, so fire and spirit resistance is worth having.
You cannot block flamebreath, so give yourself room to dodge instead.
Of everything craftable, 
Lightning Strike, 
Frostfire Sledge and 
Thunderblood Sledge put the most damage through what it resists.
Aspect of the Emerald Flame is a phantom copy of 
Fader that 
Kall Fimbulbringer summons during his second phase. It has 1,700 health, is immune to fire and spirit damage and resistant to pierce, which leaves blunt, slash, frost and lightning as the clean options. It doubles its pace when it commits, walking at 4 and running at 8, and no attack from a source other than a player will register on it.
Immune to fire and spirit, resistant to pierce. A Frostfire weapon keeps only its frost half here, and a bow is close to useless. Its bite is 180 pierce, the biggest single hit any aspect lands, and the flame breath cannot be blocked at all, so the shield answer only covers about half its kit.
With devcommands enabled, this spawns Aspect of the Emerald Flame in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Aspect_Fader · all console commands and IDs