This page of the Assassin's Creed Valhalla guide has a description of the fight with a boss named Eadwyna. You will find out when you face Eadwyn, what attacks the boss has and how to avoid them, as well as how to defeat her effectively and quickly.
The duel with Eadwyn takes place while completing the Saga Stone story mission. Eivor, Sigurd, and Basim, with the help of Geadrik, attack Cyne Belle's castle to defeat Lady Eadwyn once and for all.
In order to start the duel with Eadwyn with an advantage, it is worth to use the Focus of the Nornir ability and hit your opponent in the two sensitive points - right arm and left knee.
When your opponent is stunned, you will be able to launch a powerful attack, which will seriously damage the health of the boss.
Lady Eadwyn fights with her long spades, so her attacks have a considerable range. Her basic blows are signaled by a yellow flash – these can be locked or paired.
Watch out for the rune's red flash and the powerful overhead strike - this attack can't be blocked, so you have to dodge it.
Eadwyn uses some unusual techniques, such as kick after launching into the air using a spear, and can even set up mines on a battlefield. When you enter their range, they will explode, doing quite a lot of damage.
Eadwyn can also use a potion that will start regenerating her health. After using it, the opponent stands still and doesn't attack, and a white aura with different runes appears around her. You have to attack her to stop her from recovering lost health points.
Basim and Sigurd will help you in the fight, so the duel shouldn't take too long. Remember to block or avoid your opponent's blows and look for your chance to counter. In the fight against Eadwyn, the bow also works fine because she won't block the arrows.