Stormterror Dvalin is a boss that you can do once you reach at least Adventure Rank 25 and complete the Mondstadt related main story. You can only claim the rewards from him once a week, and it costs 60 resin to claim the rewards once you defeat him.
Before entering the fight, the mission shows the recommended party levels. You won't need to be at that levels for all of your party members, but make sure that your main damage dealer is at least around there and that they are strong enough to handle it.
For party members, you need at least 1 character that is good and consistent at dealing damage. Then at least 1 healer in the party, which is either Barbara, or Noelle for easily obtainable characters.
You can also eat some food buffs, either defensive foods to increase your survivability, or attack increasing foods to end the fight sooner. Make sure that you also bring some recovery foods so you can revive and heal once inside.
How to Change Platform
If you want to go to a different platform, use the small little wind spirits at the corners of the area. When gathered together, they will form a wind upcurrent for you to use and glide to the other sides.
You only need to do this once Dvalin starts to burn the platforms. This can also help dodge some attacks by being in the air, and going in between the platforms.
His most common attack is the scratch. This is easy to avoid since you can see him coming from afar, and he only attacks the middle part of the area. There is no tracking so you can stick to the sides of the platform and you won't get hit.
Occasionally he will do a bite attack. This can be fairly difficult to avoid since it attacks almost the entire platform. The safe spot is the middle of the platform and all the way back. This attack will also give you some time to attack him and reduce his shield.
Dvalin slams his claws on the platform, followed with a wind breath attack. This is also another attack that lets you deal damage to him to lower his shield. Start by attacking one of his claws, when the wind breath is about to come out, run to his other claws.
Dvalin will shoot out 4 homing projectiles at you. The camera angle can make it difficult to know when he's shooting out these projectiles, but he will always wiggle around in the air before shooting them. Go further away from him and move around to dodge them.
This attack is different than the 4 projectiles. Dvalin will fly very far from your location for a few seconds before shooting out 1 big energy ball to your direction. This deals massive damage, and has the potential to one-shot your character. If you see him flying around far from you, start moving to a different platform to avoid this.
This is considered his ultimate attack by Venti in the story. Dvalin will start hitting your surrounding platforms with fire, and will continuously deal damage to you if you stay there. He will only do this after you take down his shield at least once. You will have to move to a platform with no cracks/fire on it to make the fight easier.
Alternatively, you can use Geo element main character, or Venti's elemental skill to lift you off the ground and avoid taking continuous damage.
Attacking the Crystal
After you take down his shield, he will be in a downed state for a few seconds. During this time, you can attack the crystal that's above his neck. You reach it by climbing his neck, or just attack from afar with ranged characters.