On Zartan’s turn he will burn his last 2nd level spell. Tasha’s Mind Whip

Zartan speaks ancient words and shouts at the creature. INT 14 Save or take 8; half damage on save. 

if fail it can’t take a reaction until the end of its next turn. Moreover, on its next turn, it must choose whether it gets a move, an action, or a bonus action; it gets only one of the three.

Bonus Action: Telekinetic Shove Aranor 5’ towards me.
Move: I will run up and grab him and pull him back as far as I can.  

Roll didn’t work right

9 damage 

"Stop hurting my friends," Tuuli growls as she loads another bolt. She steadies herself and aims at the now no longer prone owl demon, pulling the trigger softly.

Move and BA: For Steady Aim
Action: Shoot owl critter with light crossbow. Steady Aim for Advantage begets Sneak Damage (but mostly for Advantage, since folks are within 5' of owl monster). In Game log:

21 to hit, for 14 damage.

IF the owl fails the save against Mind Whip:
Pax helps Zartan move Aranor back, and while doing so mutters a prayer to the warrior such that his wounds knit together a little and he is brought to consciousness. Then he moves such that he may block the owl's progress towards his friends, hoping the fiend will attack him, and not Serapio or Aranor.
ACTION: Helping Zartan move Aranor
BONUS ACTION: Casting Healing Word on Aranor who heals 6

IF the owl makes the save, then Pax will stay where he is and instead use his action to Dodge.
ACTION: Dodge
BA: Healing Word as above

Aranor wakes up in a pool of his own blood. The pain is immense. He doesn't really remember it, but the beast must've raked him with its talons, judging by the deep gouges that managed to penetrate his armor. The wounded elf moves away from the owl fiend, back toward Tuuli, and focuses on avoiding taking any more grievous blows from the owl.

Action: Dodge

Movement: Away from the owl, toward Tuuli

Bendrak charges in howling, trying to cleave the demon owl in half.

Attack: 18 Damage: 19

Menacing attack: DC14 WIS save or frightened until end of my next turn.

(Resolving chronologically...)

Zartan lashes out psychically at the demon, while dragging Aranor out of its reach. The thing glares at Zartan and grimaces in menace. DC14 Intelligence save: 18

Tuuli sinks another deadly bolt into it.

The thing grabs its head with its taloned demonic limbs and shrieks in pain and confusion.

Pax retreats, helping Zartan pull Aranor back. Aranor, awakened by Pax's healing magic, manages to get to his feet and retreat further on his own.

Bendrak charges in from nowhere, cleaving deeply into the demon bird with his greataxe. It is barely standing, and reduced to a panicked wariness. DC14 WIS Save: 21

Walt is up next!

Though Walt is resolute in ending this fiend, the Druid finds it difficult to attack this owl.  

Walt let’s put a cry “die you monstrosity!” Tears stream down his old weathered face as he launches his attacks. Walt launches a guided bolt and an attack from the Archer at the enemy.

Move: move closer Aranor to trigger their bond. 
Action: guiding bolt to hit 11 for 9 radiant damage. 
Bonus action: Archer strike to hit 14 for 5 radiant damage. 

The demon stretches its plague-stricken black wings to their full extent, and takes to the air for the first time, shrieking. Serapio, lying on the ground nearby, covers his ears in terror.

Starry Walt looses a flash of arcane light at the horrid thing, but misses.
However, his cosmic arrow, launched from a luminous bow, strikes the thing deep in its brow. It collapses instantly on the catwalk.

Defeated, the tlacatecolo lifts its owl head, looks Walt in the eye, and speaks: "Pazuzu will remember you." Then it immolates in a sulfuric implosion of black flames, and is gone.

Combat is over.

Zartan goes over to Serapio and offers him some water to drink. He says “it’s ok now. It’s over.”

"Same, girl. Same," Tuuli growls as the creature immolates. She hurries over to help Aranor.

Walt watches the owl fiend fall from the sky and he is overcome with a range of emotions. When the monstrosity mutters his final words, Walt's mood settles on resolve and he watches as it bursts into flames. The old Druid replies "And I'll be ready for you, Pazuzu." He turns and runs over to his friends, tending to anyone who needs aid. 

Pax also bends over Serapio, casting some more healing magic on the young man if necessary.

(Casting Healing Word on Serapio)

"Let's get him back to his mother," Tuuli suggests about Serapio, as she helps Aranor.

"Well, that didn't go quite as I expected," says Aranor as he gets painfully to his feet, clutching his side where blood is still oozing from the owl fiend's talon attack. "Yes, let's get him back to his mother."

Serapio is dazed and in shock, but able to stand and walk. "Thank you thank you..." he says quietly. "Is my mom here?" he asks, confused.

The smell of sulfur begins to fade, and the distant sound of revelry from the streets can be heard in the distance.

Zartan says "Come'on kid. We'll take you to your mom. She will be overjoyed to see you." 

Tuuli will lead the way out of the building, watching for any generic goons.