Hello, I am posting this Suggestion of Armor, Equipment & Skill combination so the you can use this to test against Valkyrie Signur

I beat Valkyrie Battles with Level 5 equipment by just playing the Mechanics but this Guide will help you play with ease of mind

Never try farm any of the equipment. Just finish the main game and Nilfheim you will have most of the items needed. Valkyries are end game content so play after after you have completed the Game. You can use this suggestion to fight all Valkyries with peace of mind just that few sockets items may need to be adjusted accordingly.

Equipment

Chest Armor

Ivaldi's Cuirass of Endless Mist

Sockets - Ivaldi's Corrupted Mind, Perfect Eye of the Realm, Perfect Aspect of the Realm

Wrist Armor

Ivaldi's Guantlets of Endless Mist

Sockets - Perfect Aspect of the Realm, Eye of the Outer Realm, Eye of the Winged Chosen

Waist Armor

Ivaldi's War belt of Cursed Mist

Sockets - Andvari's Soul, Flawless Crest of the Realm, Tyr's Shard

Note - The Sockets are what i felt best for the battle but you can customize with your own preference. Only thing make sure they add + to the stats and not -

Upgrade all Armor to the highest

Talisman - Talisman of the Realms - This is your biggest escape item. Activate this and it slows down time for you to get out of trouble. Best used as a defence and not advisable as for offensive

Atreus - Legendary Runic Vestment

Equipment & Skill

Leviathan Axe - Most recommended. Blades of Chaos can put you in extended animation risking heavy damage by Valkyries
Minimum Level 5 but i have beaten with Level 3. Make sure to Upgrade to 6 to be safe

Light Runic Attack - Hel's Touch - The best ability in the Game. Stuns & Interrupts the Valkyrie useful as both Offensive & Defensive with just 22 sec cool down. A life saver in many situations. Upgrade to max

Heavy Runic Attack - Frost Giant's Frenzy - Choose anything you like but my Suggestion is this. Upgrade to max

Axe Pommel - Mistbourne - generates health and increases Runic. This is a Must have

Atreus Runic Summon - Bitter Squirrel is Best & Safest. Wrath of Wolf good for Damage but not recommended.

Resurrection Stone - Health resurrection best. Buy it from Sindri Brok Shop

Some useful tactics -

Always go to Valkyrie with full health & spartan rage. Muspelheim Warm up activation will give this. Cancel it instantly.
Get all Idun Apples & Blood mead Horn so that you have Max Health and Spartan rage

At the Start of the Battle get close to Valkyrie and hit Hel's Touch Stunning her then give *3 attack with Leviathan Axe and nothing more
Pull back shield up with some distance and watch the Valkyrie attack to dodge roll, interrupt, shield block or side step accordingly.
Keep moving with Valkyrie insight. Use heavy runic attack only when the Valkyrie is not attacking.

if you keep over committing to an attack the Valkyrie will wipe you out in instant. Just play the Mech. You can beat the entire battle with just Axe hits and Hel's touch if you only play the Mech as long as you interrupt the large AOE attack with Atreus arrows

if your health goes down by half use the spartan rage but be careful with Signur she will wipe you out even in spartan rage mode if she throws the Ice Crystal attack. Use Bitter Squirrel to get the spartan rage or health drops.

Use Talisman of the realms ability to stop time to pick up the health drops. You can also use it at tricky stressful situations to escape.

When the Valkyrie health is low don't rush and try to burn the boss. Just play the mech as there is a chance you could get wiped in an instant with some large AOE attack or a grab attack. If you fail just keep trying and you can easily become familiar with the Mech

Hope this is useful

Good Luck! Cheers! Have Fun!