I used Sindri's Dwarf armor which is highly focused on empowering your runic attacks, and it honestly made the rest of the Valkyries a piece of cake (compared to the first two I defeated without the armor). It grants you protection while your doing a runic attack and has high cooldown so you basically always have an attack available ( You should be switching back and forth between weapons while your Runic attacks charge). I was basically spamming runic attacks and cutting through Valkyries like they were nothing. Sigrun was a bit different though.
I started out going overly aggressive on her, and while it got me close, I wasn't getting consistent results. I beat her at "barely" level 7 on normal and after trying different strategies this is what worked. You're gonna have to play a reactive fight. Watch her attacks, and learn them. Wait for her to come to you, and capitalize with Runics when she close and open after you've either counter, blocked, or evaded her attacks. I used the Realm Shift talisman, which slows down time when you dodge perfectly. Also Atreus Wolf summon was pretty useful, you keep her in place while you either go in on her, or decide to pick up health.
After you get the pattern down, you'll realize that it's not really that hard, especially if you have a Beserk Resurrection stone. Even doing pretty badly you should be able to get her halfway down, use spartan rage when you're at your last bit of health to get maybe 1 or 2 more chunks out her. Then if you die, you've got another Spartan Rage ready to take 2 more chunks of health out. When that's over you've got 4 full runic attacks and Atreus summon still left.
The most annoying part was the drop attack, which she'd sometimes do while I was in the middle of a long Runic animation and I'd have no way to evade it.
Edit: I would really love if you could fight the Valkyries again and again.