Reminder: I tried to use as less Action Cards as possible so you can follow the steps easily (That's why I sacrificed the Elemental Resonance: Woven Flames to switch the colors of the Die instead of using it). But if you have good Action Cards, you can use them if you want, as long as it doesn't disturb the instructions I gave below.
Dvalin loves to attack the next standby / previous standby characters instead of the active character in addition to the ridiculous huge buff it gains by inflicting Total Collapse debuff on our characters. But surprisingly, it was easier than I expected because I won it on the first try using the same deck as Ballad of the Scorching Loremaster.
Sharing Code: AdAg4xoOATAw8U4PBBDg8k8PBEDw9rQPC2BA97UPC3FQDbcQC9GADsMQDOEwD80QDPAA
The good thing about Dvalin here is even though he is a boss, he can still be afflicted with elements and reactions because he's not like Thunder Manifestation which keeps having Electro on itself.
About Dvalin's Card, it's pretty confusing so I will try to simplify it as much as I can.
Your first active character will have Collapsing Platform status right from the start. In the next phase, this status will attack Total Collapse to your active character. This status will make your inflicted character take +2 DMG incoming Physical or Anemo DMG for 1 turn. Dvalin can also inflict this status using his Skill.
What's not fair is that Dvalin's Weapon Card will make your next opposing standby character (left of active character) gain Total Collapse status if the current Total Collapse on active character disappears.
In short, your characters will always gain Total Collapse status = more damage taken.
First of all, start with Fischl as the starting character. Make sure you obtain at least 2-3 Pyro Dice and 2-3 Electro Dice (you can sacrifice some cards). If not, restart the challenge until you get the dice I mentioned. Follow the steps below after that:
Fischl's Skill > Switch to Bennett > Bennett's Skill > End the Round
Bennett's Skill > Bennett's Burst > End the Round
Notes: If you completely follow my steps, you should have 1 Die remaining. Do NOT use that Die to switch character at the end or Dvalin will change target to attack and you can't follow the steps below completely.
Switch to Xiangling > Xiangling's Skill > Switch to Fischl > Fischl's Skill > End the Round
Switch to Bennett > Bennett's Skill > Bennett's Skill/Normal Attack > End the Round