Roxana keeps her bowed pointed at the Cracked Horn creature. She stays in place, taking careful aim (Bonus Action - Steady Aim).
(Shortbow Attack: 19, Damage: 6 + 6 Sneak Attack) 

Round 4

Roxana's aim remains steady indeed... The arrow hits Cracked Horn just above his right eye, a blow that could have seriously injured him, if it weren't for the supernatural resistance of his skin! But even so, the creature's annoyance is evident from his grimace of pain and how he brings a claw to rub where the metal tip has chipped his skin... causing even less blood loss.

Gaer, meanwhile, has gotten close enough to make good use of Sixefsag, the magical silver blade found in the secret prison of the Harpells! The tiefling ranger joins Aidwyn and delivers a quick diagonal slash to Streak Shoulder's chest.

Gaer's Attack: 9 Damage: 8 (magical slashing)

Round 4

Streak Shoulder's gaze is comically incredulous as the enchanted blade slices through its rock-like skin with ease! Blood gushes from the monster's chest, its face twisting in pain... while its legs seem ready to give out at any moment!

Aidwyn, Rash, Anita... and Spira? (she took a Bonus Action - she can also take an Action and/or use Movement, if she likes) are up! Any order.

Combat Stats (those interested can peek):

Rash sees poor Darvin fighting alone, and decides to give Cracked Horn a real fight. (Rash should have just enough movement to reach Cracked Horn)

"Hold on, Darvin," Rash announces. "I'll take care of your little monster problem..."

With his bonus action, Rash casts Magic Weapon. And with his action, he brings the staff down HARD on Cracked Horn's ugly face, calling upon thunderous power to make sure the monster suffers if it retreats...

Booming Blade Staff

Attack: 21 (+1 from Magic Weapon); Damage: 9 (magical bludgeoning, +1 from Magic Weapon).

If Cracked Horn willingly moves 5 or more feet, it takes an extra 6 Thunder damage.

Round 4

Cracked Horn underestimates the wizard who, so uncharacteristically, instead of using amazing spells attacks him with a stick... but the dry sound of said stick hitting his wing, probably damaging it, teaches him to give less importance to appearances!

The monster's cry of pain is followed by angry sounds that must be a sequence of insults in an unknown language.

Meanwhile Anita points her crossbow at the same target chosen by Rash and pulls the trigger: "Way to go Rash! Show him that you don't mess around with half-orc wizards!"

Anita's Crossbow Attack: 10 Damage: 7 (piercing) + 2 (sneak attack) = 9

Round 4

Anita's bolt also reaches a wing, but the non-magical tip is easily deflected.

Aidwyn, meanwhile, galvanized by the help received, tries to finish Streak Shoulder with a high feint from the left cleverly transformed into a horizontal slash from the right at waist height...

Aidwyn's Longsowrd Attack: 19 Damage: 8 (piercing)

Round 4

Streak Shoulder, weakened by the wound he recently suffered, falls for the expert warrior's feint, moves his arms to defend himself from the left, only to be slashed in the belly from the right... The blade, although not magical, hits home with such power that it inflicts a deep wound, from which blood immediately flows copiously.

The monster collapses to the ground with a groan, covering his vital fluids that spread and disperse beneath him. Then he falls silent. Dead.

Dawnborn tries to reason with the surviving monster: "Surrender, you have no chance of beating us! And we are more interested in interrogating you than killing you..."

Maybe Cracked Horn doesn't understand, maybe he simply prefers a different solution, the fact is that the creature takes flight, taking care not to lower its guard as it takes off, and begins to head away from the ruins, along a diagonal trajectory that takes it to a height of about 20'.

Now Round 5 would begin... But Cracked Horn has moved out of melee range and seems determined to flee far away. If the party lets him escape, combat is over. If someone in the party wants to perform ranged attacks against the fugitive... feel free to do it, any order.

Combat Stats (those interested can peek):

Whether he disengages or not, Cracked Horn should still take 6 points of thunder damage from booming blade.

Anyway, Rash is willing to let the monster go if it's done doing whatever evil things monsters do.

"Hope you learned a valuable lesson, pal," the half-orc growls at the retreating beast. "Alright, everyone alright on our side? And is there any chance we can save that merchant with a healing spell or something?"

(OOC: Yes, I know that monsters and NPCs don't usually get death saves, but I think that's up to DM fiat whether they do or not).

[[ OOC: Oh, OK, right, thank you for reminding me of the sonic damage, I'll take it into account - but it is not enough to kill the monster, so she is still fleeing.

But as far as Fonkin is concerned, Spira already tried to toss a healing spell at him - with no effect - so he must be dead... and if someone comes closer to check, they have confirmation of it ]]

Roxana turns as she sees the Cracked Horn creature fly up and try to flee.  She again aims her short bow carefully at the target (Bonus Action - Steady Aim) and fires. 
(Short Bow Attack: 14, Damage: 6 + 5 sneak attack) 

"I see no reason to just let a creature like that just run off," she says to the others. 

Round 5

The Shadow Archer's arrow seems accurate as usual, but it hits Cracked Horn's hide at too wide an angle and simply bounces away with no effect.

Gaer sheathes his magical silver blade and holds out his Longbow: "Well said, Roxana! Who knows who his next victims will be if we just let him go like this..."

Anita, on the other hand, agrees with Rash: "Let him go... He'll soon be out of range anyway."

But Gaer's arrow is already whizzing through the air...

Gaer Longbow Attack: 14 Damage: 7 (piercing)

Round 5

Gaer's arrow fares identically to Roxana's, perhaps due to the fugitive's frantic flight movements.

As mentioned... If anyone else wants to land ranged attacks, go ahead.

Rash sighs... he really doesn't like killing things when he can avoid it, but Roxannna has a point. He half-heartedly tosses a ray of frost at the fleeing monster.

Attack: 7. Not bothering to roll damage for that.

Round 5

Rash probably usually aims with his heart, because the half-heartedly tossed magic ray misses the target completely.

Cursing the party in some unknown tongue, Cracked Horn flies at full speed, heading south in a straight line for another 120'.

If anyone still has ranged attacks that can reach him at that distance and wants to try to take him down, feel free to roll. Else... end of the fight.

Determined to at least try to stop the creature, Roxana again holds her position (Steady Aim) and fires at the Cracked Horn devil from distance.
(Shortbow Attack: 17, Damage: 11 + 9 sneak attack; note - Disadvantage from distance counters Advantage from Steady Aim for a straight roll; shortbow max range is 320') 

Gaer imitates Roxana and his arrow also suddenly shoots towards the distant target...