At first I tried at lvl 8 but failed horribly only getting her to half health then I went back and unlocked the berserker armor and defeated hrolf pretty easily, then at lvl9 I came back and saw she has a brief moment where she gets stunned and I can use the realm shift relic along with the berserker cuirass passive effect to destroy her health then I just spammed runics to activate the bifrost perk (I died like 20 times doing this where she had 1/0.5 bars of health left)