Initiative:

  Flaming Skull: 18
  Quinn: 12
  Jurgen: 11
  Boona: 8
  Dwarven Zombies: 4
  Bryony: Roll initiative!
  Ruven: Roll initiative!

If Bryony or Ruven roll an 18 or higher, they may act first.

Init: 7

Init: 13

The flameskull screams with maniacal laughter. 

A bright streak flashes from its eyes and blossoms with a low roar into an explosion of flame, enveloping everyone except Boona (but including Draco.)

Everyone except Boona must make a DC13 Dexterity saving throw. Take 31 fire damage on a failed save, or half as much damage on a successful one.

Then, everyone may act.

Five of the animated corpses are also hit: DC13 Dex saves: 2, 2, 3, 4, 15

Four of the dead dwarves are incinerated; a fifth is badly charred.

Everyone may act once they roll their saving throw (except Boona, still prone and 25' behind the last zombie.)

Jurgen seeing the ball of flame heading to him quickly casts Absorb Elements giving him resistance to lessen the blow. 

Dex Save 16

Draco also is able to miss the majority of the blast (Dex Save 23 - NAT 20)

Jurgen will move forward and attack the closest Zombie with his Longsword. (27 to hit with 18 damage <16+2 from Absorb Elements>) <NOTE: if the Zombie survived, he has disadvantage on his next attack>

He will look to his friends and see if any are in need of medical assistance

He sends Draco 20' forward towards the flaming skull and instructs him to do a self repair. 

Dex save: 19

As Drao charges forward, Jurgen scrambles out of the channel after the nearest zombie, slaying it. (It tries to reassemble: DC23 CON Save: 6)

Jurgen can dimly see the rest of the cavern together with the blast furnace and the mechanical bellows. The furnace is cold and dark, but heaps of coal are piled nearby, along with carts full of unrefined ore. A few withered corpses lie scattered around the room—individuals still wearing remnants of armor. 

The old waterwheel is partially aflame from the blast.

Bryony, Quinn, Ruven, and Boona may act, if they're up. Bryony and Ruven should roll their Dex Saves first.

Quinn shields himself and avoids taking full damage. His beard is singed badly and shrivels up a bit. Quinn sneers at the skull and roars. No one touches his beard.

He marks the skull as his prey and charges at the flaming skull and strikes down on it with Lightbringer.

22 to hit for 11 damage. (9 regular, 1 from Lightbringer against undead and 1 from hunters mark)

DM: hunters mark gives Quinn insight into his marks immunities, resistances and weaknesses. Does the flaming skull have any immunities or weaknesses?

Dex save: 6

Taking the full force of the blast, Ruven gets knocked against the wall and falls unconscious.

Boona squeezes through then stepped up behind Jurgen. She reached an arm out towards Ruven enveloping him in a green glow.

Ruven- Healing Word- 7

(Is Boona climbing out of the channel (with a Dash) or remaining within?)

Ruven is awake, but prone.

Quinn senses that this being is resistant to Lightning, Necrotic, and Piercing damage, and immune to Cold, Fire, and Poison (as well as to being Charmed, Frightened, Paralyzed, Poisoned, or Prone). It has no vulnerabilities that he can detect.
He smashes it with his glowing mace. Bit of flaming bone fly. Screams of laughter shift to screams of pain and fury.

Bryony may Save and Act.

( Dash)

Three dwarven corpses stumble and fall into the channel, under the flameskull, near Quinn.

Standing up, two abreast, the first pair begins to pummel Quinn:
Zombie #1 Slam Attack vs. Quinn: 18. Hit: 7 bludgeoning damage.
Zombie #2 Slam Attack vs. Quinn: 14. Hit: 6 bludgeoning damage.

The flameskull sets its empty, fiery eyes on the biggest target: Boona.
Fire Ray Attack #1 vs. Boona: 8. Hit: 11 fire damage.
Fire Ray Attack #2 vs. Boona: 18. Hit: 9 fire damage.

It then levitates 20' above your heads. (Quinn may get an opportunity attack as it does.)

Bryony should make a DC13 Dexterity saving throw against that fireball. (Take 31 fire damage on a failed save, or half as much on a success.)
Everyone may act!

(Hoo boy - two crits!)

Zombie vs Quinn: +2 bludgeoning damage
Fire ray vs. Boona: +6 fire damage (ouch!)

Dex Save: Nat 20 + 6

Bryony dodges out of the way of the worst of the fireball but between that and the flaming ray she took from the skull earlier, she is burned pretty badly. She grits her teeth against the pain and sets her eyes on the flaming skull. She takes aim at it with her shortbow and unleashes an arrow into one of its eye sockets.

Shortbow Attack: 19 to hit, 6 damage, plus 8 sneak attack damage

Opportunity attack:

22 to hit for 23 damage ( 10 + 5 hunters mark + 4 Lightbringer bonus against undead+ 4 colossus slayer)

Quinn fends off one of the dead zombies, but he’s too late to defend against the other. He groans in pain and lurches forward. As the flaming skull begins to levitate Quinn aims to hit the thing.

“Oh no you don’t you little bastard”

Quinn takes a wild swing and buries Lightbringer into the undead flaming skull.