The gnome leaps out from the doorway and exhales fire from his open mouth, consuming Seldor, Merzin, and the remaining zombie in the white-hot flames. Dexterity ST!

Zombie Dexterity ST: 18

Dex Save:21

 Seldor will also burn an Inspiration to use Cutting Words on the gnome and reduce the damage.

"Hey! Shouldn't you be protecting someone's lawn right now?"

3

Dex save: 19

Merzin and Seldor each take 3 fire damage.

Aegean!

 Seeing the enemy team appear in the door, Aegean taps his sword, starting a Bladesong, then circles the wight to flank with his allies and stabs at it, booming energy wreathing the blade, hoping to end it quickly so they can deal with the other adventurers without it at their back.  He then continues circling so that the wight is between himself and the other team.

BA: Bladesong

Booming Blade at the wight.  Attack: 8 Damage: 4, 7 damage if it hits and he moves on his turn.

Aegean swings wide of the mark, and the wight counters with an arrow aimed at the wizard's skull. Attack: 7 Damage: 8 Turning away from Aegean and Sssyd for a moment, the wight proceeds to launch an arrow at the gnome wizard. Attack: 6 Damage: 3

The remaining zombie lurches towards Seldor. Attack: 20 Damage: 4

The enemy druid, in brown bear form, launches himself into the fray.

The brown bear lumbers over to Seldor and bites him in the arm for 10 damage. Seldor neatly ducks a second blow from the bear.

Seldor is up!

Is Seldor able to cast Thunderwave on the bear without hitting any allies?

Seldor would hit Merzin, the zombie, the brown bear, and the gnome.

Quote from JJWattaker99 >>

Aegean swings wide of the mark, and the wight counters with an arrow aimed at the wizard's skull. Attack: 23 Damage: 10 Turning away from Aegean and Sssyd for a moment, the wight proceeds to launch an arrow at the gnome wizard. Attack: 6 Damage: 6

The remaining zombie lurches towards Seldor. Attack: 15 Damage: 4

The enemy druid, in brown bear form, launches himself into the fray.

(Shouldn't the wight have disadvantage on the attack if he's shooting at me in melee range with a ranged weapon?)

(I was searching for that rule recently but I couldn't find it anywhere. If the wight does indeed have disadvantage here it is. Attack: 17)

Quote from JJWattaker99 >>

(I was searching for that rule recently but I couldn't find it anywhere. If the wight does indeed have disadvantage here it is. Attack: 17)

 (It's in the Combat rules: https://www.dndbeyond.com/compendium/rules/basic-rules/combat#RangedAttacks.  A 17 to hit from disadvantage does not hit me with bladesong active)

(Thanks for the clarification.)

Can Seldor ready Thunderwave for after Merzin moves?

Sure, if you want to waive your turn until after Merzin's.

That means Merzin is up!

Merzin will move out of range of the spell, but where he is still able to shoot the bear. Then he will shoot the bear. (If Merzin needs to get into long range and get disadvantage, he will.)

Attack: 8 Damage: 8         (disadvantage from long range if necessary)  11

Seldor casts Thunderwave

DC13 Con Save or take 17 points of damage and be pushed back 15 feet. 

The zombie swings a rotting fist at Merzin as he steps away. Attack: 11 Damage: 6

Merzin stumbles and misfires, and the bolt ricochets off the stone wall.

Seldor casts his spell and the room explodes. Stone rains from the ceiling, dealing all enemies and Seldor an additional 1 bludgeoning damage.

Zombie Constitution ST: 6

The zombie is flung against the wall and disintegrates into a jumble of bones and festering flesh.

 While the wight is distracted, Sssyd will swing again with his scimitar.

Attack: 5 Damage: 10