Abishai's Dragons of Icespire Peak

Meira goes along with Zephyros says. She fires a shot from her bow, trying to keep the creature distracted to all Zephyros a better opening to attack. 
(Shortbow Attack: 10, Damage: 7 + 6 sneak attack; using Master of Tactics to provide HELP to Zephyros for Advantage on his next Attack) 

She then moves back along the passageway until she reaches the spot where Rasziel is located. (Moving 25 ft.) 

Alisande looks extremely disappointed when it transpires that the barrels are less combustible than she hoped, but that expression is eclipsed by the sigh of dismay that follows when her brand new vehicle is immediately ripped to shreds.

"Ugh. Well, that's just impolite." She scowls at the mimic, ejecting herself from the top hatch and vaulting onto her broom. In the next instant, her sphinx-marked blade is drawn, thrumming with the songs of mages from eons past. Astride the broom, her movements become as fluid in concert with the melismatic rhapsody that reverberates from the shortsword, warding her from harm.

The wizard wheels away from immediate peril, but the look of frustration does not abate in the slightest. She sighs and flees in the same direction as the rest of the party. "I suppose we shall have to do this the old-fashioned way."

BONUS ACTION: Bladesong.
ACTION: Disengage.
MOVEMENT: 50ft back down the tunnel. 

The young dark-haired purple-robed man with the sinister-looking scar is more annoyed tha anything else about the weird wings sprouting out from his back. It seemed this particular aberration would be tainted with wild magic essence. Moving a bit more awkwardly now, Rasziel moves closer to get a clear shot at the barrel monster before unleashing another purplish beam of hellish crackling force towards it.

Eldritch Blast: 15 Force: 8 

Meanwhile Azazel stays in the safety of his masters purple robes after dodging the weird wings sprouting from his masters back.

Meira fires another arrow before turning to run back down the hallway. The arrow just barely manages to hit the monster as she yells at it to distract it. [[OOC: Zephyros, you can roll again to see if you crit.]] The bearded veteran again throws his trident and hits it in the middle of its toothy maw. tendrils begin to wrap around the shaft just as the weapon is recalled to the warrior's hand. Alisande, ejects from the barrel crab and flies above her allies back down the hallway while Rasziel steps forward to get a line of sight on the monster to unleash another hellish blast of energy. Each attack does more and more damage to the creature. It's beginning to look a little rough.

Gareth heeds the direction from Zephyros and dashes down the hallway past his allies, towards the ballista, stopping just short of the weapon emplacement. Meanwhile, the barrel monster continues to advance 15' closer, and then 15' more to stay within range of the group as they tactically retreat down the hallway. Pseudopod tendrils grasp the floor, ceiling, and walls as it lurches forward.

Meira and everyone else is up!

Map:

Watching the barrel monster lurching towards him, now uncomfortably close, Rasziel unleashes another purplish beam of hellish crackling force towards it before quickly retreating towards the gnomish war machine.

Eldritch Blast: 26 Force: 7 

Meanwhile Azazel stays in the safety of his master's purple robes, curiously peeking at the weird wings at his master's back.

Meira frowns a bit as Rasziel pushes past with his odd new wings.  Now out of position without line-of-sight, she has no choice but to continue her retreat, unable to take a shot or to help out. After having gone 30 ft. to get just to the right of the ballista weapon, she knocks an arrow.  Turning, she draws back the string and waits until the Monster comes into view to fire. 
(READY ACTION - when Monster becomes visible - Shortbow Attack: 23, Damage: 8 piercing + 6 sneak attack; sneak attack only applies if another ally is within 5 ft at the time) 

(OOC - I don't believe Meira could be in the spot she's shown on the map after her last turn. But no need to change anything, I'll just go with how it is shown.) 

[[OOC: @Meira I apologize if I made an error. I would say you could take a shot at the monster from around the corner if you wanted to. A readied action is fine with me as well. Your choice.]]

Alisande quirks a brow at Rasziel, watching as bullets of arcane force burst from the butterfly-winged man. "Another mysterious gift of yours?" The wizard calls as she sweeps her way into the small chamber with the multi-directional ballista.

"Would you care to do the honours, Gareth?" She declares courteously, gesturing to the seat fixed at the centre of the contraption. As the wizard hovers over the device, she peers at the levers and pedals, ready to impart advice on its operation. In the event that a monster materialises before them, she ignites a bulb of arcane fire at any unfriendly creature that dares approach.

MOVEMENT: 15ft directly above the central seat.
ACTION: If the contraption seems at all complicated, she will give Gareth the Help action, whether that's for an Investigation to determine how it works, or on an attack roll to grant advantage. Otherwise, she will Ready a Fire Bolt. To hit: 8 (Natural 1). Damage: 2

Gareth's eyes widen at the sight of the wings sprouting from Rasziel's back. There's no time to stand around thinking about it for too long though because the barrel monster is chasing them down the hallway. The ranger and Alisande are the closest to the crossbow contraption and Alisande gestures him toward the device. He shrugs, hopping up into the seat. Glancing down at the controls, he tries to get a sense of how the levers and pedals operate so that he can point the thing toward the entrance where the barrel monster will surely appear any moment.

Action: Investigation to see how it operates if necessary - 18 (second roll with Help was a 4); otherwise he will ready an action to fire the ballista at the barrel monster as soon as it appears.

(Second attack roll with advantage is 19)

Zephyros raises his shield high and crosses his trident across himself for added protection as he continues to retreat down the corridor towards the ballista. He then drops low to give a clear shot to anyone operating the heavy weapon. Once the monster is in sight, the bearded veteran yells, "NOW!"

Action: Disengage

Movement: 30' towards the ballista 

Meira continues into the ballista room and prepares for the monster while Gareth and Alisande quickly take a crash course on the ballista's operation. Gareth is able to figure out the pedals move the ballista left and right and the levers fire the bolts in quick succession. Zephyros avoids the pseudopod tendrils as he continues to retreat down the corridor, while Rasziel blasts the monster once more and joins the rest of the group.

The monster's pursuit is relentless as it continues to dash after Zephyros, driven by a primal hunger, sensing the meal just out of reach. As Zephyros drops low and yells to his companions, Meira fires, sinking another arrow deep into the creature. Alisande positioned above the ranger, signals the opportune moment to fire. Gareth pulls the lever to fire the ballista...

[[OOC: Gareth, please make an attack roll with advantage 4d10 damage on a hit.]]

Map:

Gareth swivels the crossbow contraption to face the barrel monster. He grins as he fires the massive bolt at the monster.

Attack: (I added just the DEX modifier to the attack roll since I'm assuming he's not proficient with a giant crossbow ballista machine lol) Oh my, I just rolled a NAT 20 with advantage! I'm assuming that means 8d10 for damage...44 points of damage! (Rolled in the campaign log.)

Gareth, [[Awesome!]]

You activate the ballista which whirls underneath you in a blur of motion. Four ballista bolts fire off in quick succession just above Zephyros' head. The bolts tear into the monster causing massive damage. The ballista auto loads four new bolts into the firing mechanisms. 

The barrel monster recoils as the bolts pierce it. The monster wraps tendrils around the bolts and begins to pull them out, but isn't able to before collapsing to the ground, dead, in a massive heap of bubbling acid.

Combat has ended.

"These ridiculous wings that just teared through my robes? Most certainly not, just some annoying arcane side effect that I can't quite explain." Rasziel responds to the red-haired lady as the barrel monster is finally put down by Gareth and the ingenious gnomish war machine. "Please no more comments about these." He tells the others, seeming more than annoyed about the wings on his back. "Hopefully the transmutation magic will dissapate in time, until then we should proceed to get this over with."

"I've got to get me one of these," Gareth exclaims as the barrel monster is obliterated by the ballista. He hops down out of the machine with a huge grin on his face. 

"Thanks for letting me do the honors," he says to Alisande, watching her fly around on her broom. 

At Rasziel's request, he doesn't comment on the man's new pair of wings. 

"Good shot, I say! The kind of shot you can most assuredly regale Sylvie about, without even a jot of embellishment." Alisande laughs, giving Gareth a slow clap, equal parts astonishment and adulation. "Oh, think nothing of it, my dear. I had no doubt you were equal to the task."

She zips back and forth on her broom, then glances over at Rasziel as he expresses his irritation. "Perhaps he's just evolving into his final form?" She murmurs under her breath, amused if not particularly troubled by the merchant's wondrous metamorphosis. That is, until her attention is commandeered by the quarrelsome wand tied to the loop on her hip. The magical implement which refused the authority of its wielder and disgorged whatever unsanctioned arcane caprice it pleased. 

She frowns thoughtfully, and then turns back south, resuming the mission of exploring Gnomengarde. From time to time, she weaves small illusions ahead of their path, baiting any hidden mimics into revealing themselves - the plushie of Ozymandias, a juicy purple mushroom, a miniature barrel crab.

ACTION: Minor Illusion
Perception: 12

"What a shot Gareth!" Meria says excitedly as the threat from the monster is ended. "I mean, I'm sure without my shot to soften it up it wouldn't have been as effective," she adds with a grin. "But you seemed like an expert with thing!" 

She glances over again at Rasziel and his new wings. She had been going to complain about him getting in the way in the narrow passage. But it seems he's frustrated enough by the appearance of the wings. In an unusual turn of events, she just keeps quiet about it. "Well, let's catch our breath and then continue on. Anywhere else we need to check out? I hope there aren't any bigger ones left lurking around!" 

Zephyros gets back up from ducking under Meira's arrow and the ballista bolts. "Whew what a weapon!" The battle hardened veteran states. "If we had these in the war it would have ended with far fewer dead." The warrior walks over to the weapon with an excited reverence, running a hand over its contours. Taking as much detail in as possible. He stops as Meira expresses hope that was the last of their adversaries in Gnomengarde. Ironheart glances at Rasziel's butterfly wings, smiles, and lets out the softest of laughs before asking, "Heh. You want me to cut those things off? I can mend those robes afterwards if you'd like."

Turning to Alisande, Zephyros adds, "Great idea to run that contraption straight into the monster. It seems it slowed it down perfectly."

Glancing at the giant blob of acid, the warrior begins to analyze the fight, 'That tactic worked perfectly. Not a single person hurt and one monster dead.' He looks for a safe route past the monster to continue their mission. "Yes, let's keep going. We still have more to do here. Stay vigilant."

Investigation: 4

Rasziel too gives the ranger an acknowledging nod over his skill with the gnomish war machine, then turning to the bearded veteran. "If they don't disappear soon I might ask you to do just that. And the offer of mending of my robes is most welcome." He says with a polite nod, then ready to, albeit a bit awkwardly, continue the sweep of the eastern parts of Gnomengarde. 

Perception assisted by Azazel: 9

Alisande & Rasziel,

You can fly over the huge pile of bubbling acid in the corridor without difficulty to continue your exploration.

Everyone else,

Please give me a dexterity saving throw to safely traverse past the dead monster. DC 15 or take 6 acid damage as your bare skin touches the acidic blood. Or you can choose to go a different route or come up with a creative way to overcome the obstacle. 

 Zephyros,

You don't see a good way past the bubbling acid in the hallway.