If you beat the previous Valkyries, she shouldn't be too tough. I beat her on my 2nd try. She basically has the same patterns as the others.
Just be patient, learn her attacks, and don't get impulsive.
Use Spartan Rage as soon as you're half health.
Use all of your runic attacks from the start.
Use the squirrel rune for Atreyus so that you can heal yourself. Have a the purple resurrection stone so you can resurrection with Spartan Rage and immediately use it.