|Thamur's Corpse||Upgrade materials, Feather's of the Fallen Servant Enchantment|
Gunnr should be the first Valkyrie that you'll fight, these are some of the toughest optional battles in the game that require patience and skill to best. You'll need to learn the attack patterns of the Valkyries and it may take many attempts to learn it so don't be disheartened if you're defeated, you'll restart from just outside the room regardless.
Gunnr is a good starting point for these battles, she only has three main attacks and serves as a good tutorial as for what is to come. Her first attack is a Wing Combo, she'll spin her wings at you in a circular motion that you can parry or block but the final attack will be an unblockable punch so it's generally a good idea to dodge this entire combo. When she begins the attack, perform a double dodge roll back to gain enough distance.
She can also charge towards you from a great distance with a two hit Scythe Combo, this one is easy to block against so just keep your shield raised to block the attack. Finally, when she rises above the ground, be prepared to dodge roll to the side as she will swoop in for a scythe attack that will break your guard. A dodge roll or sidestep will leave her completely open however and is your best opportunity for a damaging combo.
Your best method of attack is your Runic Attacks, this paired with the Talisman of Unbound Potential is a great asset. Begin by having Atreus cast a Shock charged Murder of Crows and continuously pelt her with arrows throughout the battle. When she begins a Wing combo, you can use Charge of the White Bear to nullify the attack and stun her with the follow up attack, this leaves you a great opportunity to score some good attacks on her. Finally, use Blessings of the Frost on cooldown, this will add much needed damage and range to your attacks, bringing her health down quickly.
Always pay attention to your health, don't get too greedy with your attacks and only use Spartan Rage when your health drops low in order to use it as a health regen tool. Once she takes enough damage, she will fall to the ground, quickly dash in and perform a Stun Grab to end the battle or risk her recovering some health.
With her wings ripped off, the spirit of the Valkyrie will depart from the body and warn you of the threat you face. Take the helmet form the body to obtain Gunnr's Helmet along with a huge chunk of XP. This also begins the Legendary Favor, Prove Your Valor and is probably the game's most difficult Favor. Claim your spoils from your victory to obtain Asgardian Steel x2, the Feathers of the Fallen Servant Legendary Enchantment and another random Enchantment. Before you leave this chamber, look on top of the tree in this room to find another Raven.