Can't beat Sigrun myself. Got maxed out weapons (both), maxed out Sidri + Valk armour (Sidri Chest and Wrist, Valkyrie Waist, IIRC), focusing on Cooldown, but... nope. My best run so far has gotten me to about 4/5th of her health but I just make inevitable mistakes/am too slow to react to some of her attacks. The worst ones are the blinding and the jumping/kick spam. If my axe happens to be out while she charges the blinding flash thing, I can interrupt it, but if not I'm fucked. And the jumping kick where she knocks you down and spam kick, even if I button mash X I still take tons of damage. I can avoid it if she happens to be at a certain distance when she starts it, but if I happen to be close, I can't ever dodge it in time. Grrr
(Playing on Hard, btw.)