According to pic you have attempted some times already. Usual having best upgraded gear, best foods and meads, having rested buff and Bonemass power ready add to her too, but some extra tips:
- As she summons seekers and broods during the fight, you might want to have some weapon(s) to deal AoE damage against her adds. Bilebombs, Demolisher, Himminafl and Ember staff are effective for that.
- You can have some breather spots inside the citadel, like some corners and the entrance she cannot fit. Also floating from top with Feather cape gives you some time to regain stamina/eitr/hp in dire situation.
- If you use Bonemass power with melee gear and heavy armor you can facetank her attacks some time while dealing damage, so standing between her and a wall helps counter her knockbacks.



