Before the fight commences, ensure that you’re stocked up on Tawny Owl, Swallow and Full Moon.
During the first phase of the fight, don’t be relentless as it will only get you killed! When you see him holding his sword in a defensive stance, don’t go in with a frontal attack and try to flank him.
Having Igni Sign Melt Armour helps a ton during this fight as it allows you to decrease his defences quite early in the battle. Eredin also possess a Teleport Ability which he will use to get behind you; roll away in such situation to avoid taking damage.
I also recommend bringing Samum to temporarily blind Eredin, but you’ll have to flank him quickly since he recovers from the bomb’s effects faster than the others.
Moreover, I don’t recommend facing him head-on. You’ll notice that he not possess a high defence, but his attacks are also extremely relentless unless you counter-attack him and leave him staggering.
Watch out for an attack where Eredin raises his sword with both hands. The boss can execute this attack in almost middle of anything and you can’t parry/counter it; you can only dodge or roll away.
You can also use Yrden Sign to counter-attack, but know that he can use his Teleport Ability in the middle of Yrden Sign. The key to winning the first phase of the battle is to take it slow and don’t be greedy into depleting his Vitality in one go.
During the second phase of the battle, Eredin will get several new attacks up his sleeve. His Shockwave Attacks can be dodged by simply running and taking the time between his attacks to deliver a blow before backing off again.
However, do note that you may face difficulty dodging the Shockwave Attack if you’re too far from the boss.
As for his Meteor Attacks, I found that the best to evade them is to get real close to Eredin and let his attacks hurt himself which will deal a whole lot of damage and end the attack instantly while leaving him for follow-up attacks.
As for Magical Sphere, you can also end this attacks by moving in close to Eredin and delivering a couple of Strong Attacks. At the very end of the fight, Eredin will open up a portal. Simply follow the Elf back through the portal and end this boss fight!