At the start, focus on one of them, preferably Magni because he’s the bigger threat between the two of them. Modi will keep Atreus busy so you’ll have limited support from him. If possible, try to use runic attacks when they’re both in range so you can damage them at the same time. Watch out for Magni’s long reach and unblockable attacks. Keep showering them with Atreus’ shock arrows since it can jump between targets and damage the two of them if they’re together. Modi will use his shield to his advantage so it’s more a reason to focus on Magni first.
When one of them is down by two bars, he’ll be temporarily disabled and will kneel on the ground with an aura of lightning. He’ll be invulnerable while in this state so don’t waste your attacks on him until he stands back up. Once the other is also down by two HP bars, they’ll perform a special attack called the Snowblind.
While the attack is in effect, you’ll be back to back with Atreus in a defensive position. You have to rotate to the direction of the attack and block it. Keep an eye to your threat indicator as you’ll only have 1-2 seconds to react. Remember that you don’t need to parry the attack. You can hold your shield up early in you’re already sure of the direction where they’re coming from. Continue doing this until the effect is over.
Once the battle resumes, both of them will have new attacks. Modi will be able to throw a shock “grenade” while Magni will dig his sword to the ground and slash his sword upward, creating a massive electric wave that will travel in a straight line.
They can also perform a combination attack where Magni throws Modi up in the air and the latter slams the ground with an electric shockwave. Be careful of Magni’s area shock attack. You can predict it when his sword is imbued with lightning and he’s about to stab the ground. Keep focusing on one of them until you take out two HP bars off your target. Once one of them is temporarily disabled, use this chance to pick up any healthstones around you while avoiding the other brother’s attack.
Once both of them are down by two health bars, they’ll perform their Snowblind attack once more. Like before, watch the threat indicator and react accordingly. This time, they’ll be performing fake attacks, making it a bit challenging to predict the real attack. Just put up your shield when one of them approaches you then quickly put it down and rotate when the first aggressor starts to move back. This fake will always involve a real attack from behind.
After the effect is over again, you’ll return to normal combat. At this point, they’ll be more aggressive with their attacks and combos. Like before, focus on one of them to disable him then finish off the other. They’ll do their snowblind attack one more time but this is just for dramatic effect. During this cutscene, you’ll be able to finish off Magni by simply pressing the attack button when prompted. After the scene, the battle’s over and Modi will run away.