Originally posted by Yakumo_2020:

Frostner is next to useless against Moder. She is resistent to both frost and bludgeon damge and flate immune to spirit damage.

https://www.slythergames.com/2021/02/23/valheim-enemy-weaknesses-resistances/

took me less than 10 minutes to finish her with a wolf spear or a black metal sword when I killed her a second time. Having a Stagebreaker handy helped too since the AOE from it shatters the icicles from her breath weapon.