Gammeltroll
13
What to know
Gammeltroll has 3,000 health.
It takes extra damage from pickaxe; it resists blunt, slash, pierce and frost; fire and spirit do nothing to it at all.
Its hardest hit is punch at 220 blunt, out of 5 attacks.
Of everything craftable, 
Lightning Strike, 
Thunderblood Crossbow and 
Thunderblood Sledge put the most damage through what it resists.
Spawn command
With devcommands enabled, this spawns Gammeltroll in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: TrollFrost · all console commands and IDs