Brenna has 1,200 health and spawns in Black Forest.
It takes extra damage from blunt and frost; it resists pierce; fire and poison do nothing to it at all. Slash lands at full damage, so that is what to bring.
Its hardest hit is sword hildir at 60 slash and 20 fire, out of 2 attacks.
Some of what it throws is elemental, so fire resistance is worth having.
Of everything craftable, 
Frostfire Sledge, 
Thunderblood Sledge and 
Frostfire Mace put the most damage through what it resists.
It always drops 
Hildir's Brass Chest and 
Brenna Trophy.
Brenna is an undead miniboss found in the Smouldering Tombs of the Black Forest, guarding the first of Hildir's stolen chests. She has 1,200 health, is immune to fire and poison, resists pierce, and takes extra damage from blunt and frost. Her sword swing deals 60 slash and 20 fire in a 1.8 meter arc every 3 seconds, and every 10 seconds she drives the blade into the ground for a 75 fire blast out to 3 meters. She always drops Hildir's Brass Chest and a Brenna Trophy.
She is immune to fire and weak to blunt and frost, so a good mace carries the fight. The sword swings parry cleanly; the nova around her does not, so step out when she stabs the ground. She also headlines the 'She's hot on your tail' raid after you beat her, which is her way of writing back.
With devcommands enabled, this spawns Brenna in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Skeleton_Hildir · all console commands and IDs