Surtling has 20 health and spawns in the swamps.
It takes extra damage from frost; fire, poison and spirit do nothing to it at all. Blunt, slash and pierce land at full damage, so that is what to bring.
Its only attack is fireball attack, 40 fire and 10 blunt.
Some of what it throws is elemental, so fire resistance is worth having.
Of everything craftable, 
Frostfire Crossbow, 
Frostfire Sledge and 
Thunderblood Crossbow put the most damage through what it resists.
It always drops 
Coal.
Surtling is a small fire demon that spawns from Fire Geysers in the Swamp, up to three per geyser. It has 20 health, is immune to fire, poison and spirit, and is weak to frost. Its only attack is a thrown fireball for 10 blunt and 40 fire that applies Burning, on a 3 second cooldown out to 10 meters, and rain in the Swamp cuts the fire portion considerably. Water kills it outright, so digging the ground around a geyser down to the water table turns the spawn point into a free source of loot. It drops 4 to 5 Coal and a 
Surtling Core half the time, plus a 
Surtling Trophy at 5%.
Water kills them outright. Dig the ground around a geyser below water level and the spawns drown themselves into a free core farm; every veteran has one. In a straight fight they are weak to frost and die to a stiff breeze, just do not stand in the fireball.
With devcommands enabled, this spawns Surtling in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Surtling · all console commands and IDs