„Any better ideas, anyone?“ asks Myrla with a new arrow nocked and at the ready.

Quote from Swiftgale >>

Now they cast sleep, has the dust cyclone reduced even further?

 (I assume that the "dust remaining unchanged" refers to this too? If so, the following reply is valid. If the dust cyclone did reduce after they cast sleep, Milo has a different plan)

"Is it worth those of you who can't be put to sleep go into the room to draw them out, while the rest of us attack from a distance? Take a rope with you so you can find your way back if they managed to get sand in your eyes again."

Myrla lets go of the second arrow and shoots at first sight of anything that looks like movement, then she draws her rapier and her shield and prepares to enter. "Ok, let's try Milo's idea. Inge, are you ready to hunt a few mephits? Let's approach cautiously and dodge whatever they throw at us and perhaps, they'll try to engage. With any luck we can overpower them with the help of the others from back here?" 

 Action: attack with Longbow, (17,10)+6 for 5 damage
Follow-up actions: don shield and get ready to move in side-by-side with Inge while dodge-ing attacks. (I'm including this for brevity, if something important happens before she has opportunity for this, simply disregard.)

Inge is not in love with this plan.  But then, she is angry in general right now and not thinking straight.  She wants to hit something with her glaive.

Nevertheless, she grumbles and accedes, putting away her readied javelin and hefting her glaive.  When Myrla advances into the room, Inge walks alongside her, taking the dodge action  She tries to continue to move so she is always just over 10' away from the dust cloud.  (Any mephit that appears and then enters Inge's 10' reach is subject to her opportunity attack due to Polearm Master.)

Quote from Tybard >>

(Any mephit that appears and enters Inge's 10' reach is subject to her opportunity attack due to Polearm Master.)

 (I wonder how that works if it enters your range unseen and then becomes visible when it attacks. Does it trigger the PM AoO? Or does it need to be visible at the moment when it moves into range?)

"Wait!! Wait!!! cries Brix as he pushes past everyone by just enough so that he can release some more sparkling dust into the room. 

Dex check:13 

{{I will resolve this as follows.}}

Before Myrla and Inge can step into the room again, Brix interrupts them and throws a handful of sparkling colourful dust into the dust cyclone. The effect is immediate. While the mephits are almost identical in colour to the dust cyclone, Brix's dust sparkling dust seems to stick to the mephits' form and makes them sparkle in a variety of colours behind their hiding spot within the dust cyclone. As Myrla and Inge take their first step into the room, the dust cyclone disappears just in time for both women to launch their first attack. The mephits are taken by surprise by the sudden ferocity of the two attackers. Without the cover of the dust cyclone and with their body covered by colourful dust, they are in their most visible, and therefore vulnerable, state. 

{{I will give Myrla and Inge a surprise round. Make your attack with an advantage. Inge, if you want, you can change your action from a dodge to a direct attack toward one of the mephits. I will say that Brix also has a surprise round, but he already did that by using the faerie fire. Then, I want everyone to roll initiative. I'll roll for Hunteros if we still don't have any news from burningkings in two or three days.}}

{{Just to give you a clearer description of what is happening, three mephits are grouped up in the middle of the room. They are 15 ft. away from the door. Myrla, Inge, and Brix are right at the door. The rest of the group is still in the corridor, maybe 10 or 5 ft. behind Myrla, Inge, and Brix.}}

Inge's frown changes to a beaming smile right quick as Brix's own dusty magic reveals and illuminates the mephits.  No longer angry, she does not Rage.

"Now that's what I'm talking about, Brixie!"  She grins down at the diminutive artificer.

Spinning her glaive in a tight spiral so as not to hit her companions, Inge steps forward only 5', so she is 10' away from the nearest mephit.

She favors her target with a big, toothy grin.  "See how it feels when you get dust chucked at you?  Not too good, huh, you little bugger?"  She catches one of the elemental creatures square through the torso with a cleaving slice of her glaive, continuing the arc to bring the polearm's haft into play, pummeling the little elemental for a second blow to its face from 10' away before retreating back out the door.

Inge smiles sidelong at Myrla, genuinely happy.  "Hit and run.  Or maybe just shoot them at range, now that they're all shiny.  Then step back out."

Inge's Initiative for later rounds:  6

Movement:  Inge advances 5' west to within 10' of the mephits (in range of her glaive's 10' reach).  2 attacks described below, then step 5-10' back just out of the room.  (Assuming she takes no opportunity attack since she gets no closer than 10' away from the mephits, using her reach.)

Action:  Inge attacks closest mephit from 10' away with advantage from Faerie Fire using glaive:
To hit:  26 (Natural 20)
Damage:  16 slashing (low damage roll for a crunchy crit)

Bonus Action:  Attack same mephit (or another one if down) with advantage from Faerie Fire using back end of glaive (Polearm Master bonus attack).
To hit:  25
Damage:  6 bludgeoning

Swiftly, Inge steps closer to the mephit and strikes it with the blade of her glaive. The mephit separates into two as it screeches in pain. It begins to reform but Inge doubles back to hit it with the other end of the glaive. The mephit explodes violently in its death throes. Inge is far enough to only feel a slight puff of wind. In her experience, it would not be good to be close to a dying mephit. 

{{That's one dead mephit. Nice crit.}}

Myrla keeps pace with Inge and expected to get in range and strike herself. However, Inge stopped at her range and outright split a mephit in half, which even caused an explosion, and then she started to step back. Myrla had to decide, should she go forward and strike a mephit, too? And expose herself, of course. No, Myrla decided to follow back. On the way she releases her shield, sheathes her rapier and draws, again, her longbow. “I’ll go with shooting,” she replies to Inge. “I really don’t want to have one of them explode in my face.”

Action: doff shield
Move: follow Inge inside and return with her to the door, outside the chamber
Free: sheath rapier and draw longbow
Initiative: 17

At the sign of combat, Hunteros' eyes glazed. The creak of the leather of his weapon's handle is audible as he clutches it tighter. He begins muttering something, inaudible but distressed. As one of the mephit explodes, he turns and runs back toward where the group previously came from, swinging his melee weapon around as if fighting invisible enemies that only he could see. 

Wadi's Init: 24

Brix: 17

Milo: 17

Wadi wil lattack with his sling

Attack: 18 Damage: 8

It originally said 5 damage, not 8.

Myrla steps through the others (difficult terrain) to enter the chamber. There she nocks an arrow takes aim at one of the mephits, if one got hit by Wadi then the same, and she lets loose the arrow. Without waiting to see if she hits the mark, she turns around and returns to her previous position.

Milo is much better in melee than ranged, but he does carry a light crossbow for those situations that melee is not really an option. He fires of a bolt at the same (or the final one if it is dead) Mephit, the bolt going wildly off course in the sandstorm.

Wadi aims the nearest mephit with his sling and shoots. As the projectile goes through the door, there is a brief imperceptible flash of blue around the door. The projectile strikes true at the mephit, but something is wrong. It is not doing as much damage as expected. 

Myrla steps into the room to get a better aim through the crowd by the door. Her arrow pierces through the mephit that Wadi shot. The hole on the mephit's body slowly regenerates, but the dust surrounding the mephit starts to thin. 

Following Wadi and Myrla to remain at range, Milo takes out his crossbow and shoots at the same mephit that Myrla and Wadi had shot. As with Wadi's, Milo's arrow goes through the door and there is another brief imperceptible flash of blue around the door. An errant gust of wind and dust makes the arrow go off course, hitting the wall and clattering to the ground. 

With Inge being the most present threat, one mephit surrounds her and scratches her with its clawed hand. The other mephit gives Inge a wide berth and throws a dust powder toward the group outside the corridor. Strangely, you note that while a mystical barrier on the door seems to be blocking the dust swirling in the room, the dust that the mephit throws can get out of the door. 

Mephit 1: DEAD

Mephit 2 {{The one everyone has been attacking}}: 
Movement to Melee Range of Inge
Claws Attack: 22 Damage: 5 slashing to Inge

Mephit 3: 
Movement backs away from Inge 10 ft.
Cast Sleep for 22 toward the immediate area of the corridor affecting Brix, Milo, and Wadi. Myrla is immune to this effect. 

{{Inge, your feat will pop off as Mephit 2 comes into your 10 ft melee range.}}
{{Who is the two the nearest to the door? I saw that Brix, Milo, and Wadi each has 10 hp. The two nearest to the door fall unconscious, which probably would be Milo and Wadi because they were just attacking? If Brix falls unconscious, the Faerie Fire drops.}}

Brix suddenly feels overwhelmed with drowsiness. No matter how hard he struggles he just can't keep his lids open.