"Meat shields to the front," Polly 'shouts' in a whisper.  "OwImp if going to try to draw some of the lesser ones away, we should then immediately attack the boss."

     Polly will pause to give everyone time to position themselves, and to argue a bit with OwImp who complains verily about this plan...  The below is all choreographed by Polly using her link to OwImp to get the timing right.

1. OwImp, invisible, is going to approach the boss.
2. On the GO command, OwImp is going to strike at the Boss Demon with his tail. ((Investment of the Chain Master - As a bonus action, you can command the familiar to take the Attack action.))
3. OwImp will then take off at full speed heading towards and then past d1.  Not intending to get close to d1, just spending his 40feet of flying heading down that way, trying to draw smaller demons after him.  I half expect Boss demon to order them after him...  While flying away, OwImp will be shouting insults about the demons lineage.  Just for flavor. 
4. Once OwImp can turn invisible again, he will, then fly back to Polly asap.  ((Assuming he makes it through alive that long.))

OwImp Stinger Attack: 20 Damage: 7 On a hit, target must make a DC 11 Constitution saving throw vs DC 16, taking 10 (3d6) poison damage on a failed save, or half as much damage on a successful one.  But they're probably immune to poison?

     Polly will advance behind the meat shields but stop about 50 feet away from the Boss Demon and cast Ray of Sickness at them.

Ray of Sickness Attack: 22 Damage: 3  poison damage and must make a Constitution saving throw vs DC 16. On a failed save, it is also poisoned until the end of your next turn.  ((Prolly still poison immune but worth a shot...))

     Polly will stay well behind the meat shields but try to advance within 60' of the Boss demon and cast 

With the plan set and people moving.  Adray would move faster.  With a rush of speed he closed the distance to the boss demon before the creature would know what hit him.  The mark on his chest would sear slightly and his fist turn greenish as he threw two punches at the creature.

Initiative 15

Bonus Action: 1 ki point for step of the wind dash

Move: Move up to 90 feet, and put himself to the left of the bosses current position, that should still leave line of sight

Attacks will deal acid damage

Attack 1: 29 Damage: 10

Attack 2: 18 Damage: 9

 Attack 1 will be a stunning strike, DC 15 Con save or be stunned for a round

Choir will hastily cast Bless on Ordrag, Adray and Fran.

Initiative: 7

(Or maybe not so hastily.)

Initiative: 13 

Ordrag Initiative: 14

Initiative?  How novel!  ((In other words, totes forgot that!))

polly initiative: 10

(( Reminder to Fran - by the new rules (came with Tasha) Brutus's attacks spend your Bonus Actions))

Diggers: 5, Lulu 14, Mooncolor 21

No plan survives contact with the enemy - the old saying proved to be true once again. Owimp boldly materialized right by the Boss - and even stung the demon! But instead of following the annoying creature, the demon yelled more commands "Intruders!!! DIG FASTER! FIGHT!" - and as if melted away inside the floor (you saw something like that with Dao before). 

Some of the digging demons begun to turning their ugly heads toward the yelling Imp, though it was hard to tell if they understood the words.

(Map is the same minus boss, it seems Fran and Brutus begin, but party goes first anyway. Adray and Polly can adjust their actions)

(When it's his turn...) Ordrag will likewise rush in and attack the nearest digging demon(s). Time for a break, fellas, he says to the diggers. Smoke if you got 'em. 

Action: Attack/Longsword.
Attack#1: 19 Damage: 14 slashing.
Attack#2: 23 Damage: 9 slashing.

Bonus Action: TWF/Warhammer.
Attack: 18 Damage: 7 bludgeoning.

Reroll damage for Longsword #2: 7 slashing

 ((Assuming Polly can get it off before companions get into melee...))

   With the big boss out of sight, Polly changes tactics.  “pêl dân!“ She hisses while clutching her necklace, pointing at the biggest congregation of demons left.  From her fingertip a small, glowing ember lashes out, striking the ground and exploding into a giant Fireball that envelopes all within a 20 foot sphere of it.

23 for those who fail Dex save vs DC 16, half to those who succeed...  

((Technically the order is Fran Brutus Lulu Adray Ordrag Mooncolor Polly Choir Diggers . On the other hand, she can convince others to let her to clear up some space first))

((If others engage first it’s just change her placement so as to 1. Not hit her teammates then 2. Get as many others as possible...))

(I thought Brutus always used up her Bonus Action. What I saw was new in Tasha’s Cauldron was Might of the Master:

Might of the Master. The following numbers increase by 1 when your proficiency bonus increases by 1: the defender’s skill and saving throw bonuses (above), the bonuses to hit and damage of its rend attack, and the number of hit points restored by its Repair action.)

Fran’s eyes would dart around the scene before her, positioning herself off to the side to allow the others to charge into melee. Unable to make out or follow any form of plan Polly barked out, the artificer would just let her instincts take over. With the adrenaline pumping from her little snack, she’d fire off some bolts from her crossbow and send Brutus running in.

Attack 1: 26 Damage: 13 
Attack 2: 20 Damage: 12 
Extra haste attack: 14 Damage: 11 

Brutus force empowered rend: Bonus action: 17 Damage: 11 

(Fran would position/post herself hugging the north west alcove to pick off from left to right if that makes sense.) 

With the boss man disappeared, Adray will move up, try to get as many of them in a line as he can and spit a 30ft line of lightning at the diggers.  He will then punch the closest.

Move to get as many in a 30 foot line as possible. 

Bonus Action: use step of the wind to gain a dash action if needed, total movement would be 90 ft.

Action: 1 attack converted to dragon breath lightning 30 ft. 12 lightning damage, DC 15 dex save for half.

2nd attack: Attack: 26 Damage: 7

(About Brutus - I thought so too, but in the podcast about changes they specifically emphasized BO, so I mentioned it just in case.

Let's say Polly did threw her fireball before others.)

The firestorm would be absolutely devastating if not for demonic nature of the diggers. Non fell though all suffered - the yells and squeals filled the air. But soon sounds died out - combined attack made a quick job of the burned demons - bolts, lightning breath, bites hits and slashes, and, as a grand finale - Lulu's sparkling trumpeting. 

Two remaining demons proved the devils saying "dumb as a demon" - they attacked. It was not something remotely dangerous but annoying enough:

vs Adray Attack: 22 Damage: 6

vs Adray Attack: 8 Damage: 6

vs Ordrag Attack: 21 Damage: 3

vs Ordrag Attack: 9 Damage: 7

The worst assault was the gas cloud filling the "cave" (everyone in the green circle - DC 11 Constitution saving throw or be poisoned until the start of your next turn). 

That was the moment when first, ground shaking "thud" came from the wall the demons were digging up.

Round 2

Everyone!

     “That doesn’t sound... good?”

     Polly moves in closer, but not too much closer. (Like 1 square outside the stink cloud?) She uses Magic Stone on some rocks she’s been gathering as she’s seen them and gave them to OwImp as he returned to her, for the moment hiding behind Polly by hanging from her backpack and peeking over her shoulder...

(Absolutely! Thank you :) Fran is out eh? Brutus is immune.)

Fran would pull her cowl up over her nose. She'd reload her bolts, take aim and continue to pepper the little demons. She was covering for her friends caught in the noxious gas.

Attack 1: 22 Damage: 12 
Attack 2: 23 Damage: 11 
Extra haste attack: 10 Damage: 11 

Brutus would round around to D7 to provide Adray with support, snapping at the demon.

Brutus force empowered rend: Bonus action: 26 Damage: 14 

((Oops, misread while at work and thought all little demons squashed.  Polly and OwImp actions stay the same but with some added Eldritch blasts !!))

     Not having moved fully into the cavern, D7 is likely the only one Polly can see...

D7 Attack: 17 Damage: 7

Second D7 Attack: 12 Damage: 10

Choir moves a little closer and casts Sacred Flame at whichever of the demons looks in the worst shape, for 13 damage (Dex save 17)

(Don't forget, Adray Ordrag and Fran are Blessed. I don't suppose a blessing on Fran boosts Brutus, though...maybe I should have chosen him.)

(Thank you for the reminder! <3 Rolling some bless for Fran)

Bless 1: 4
Bless 2: 1
Bless 3: 2

Adray con save 11 + 4 from bless

Adray will try to finish the digger demon in front of him and if taking one out move to the other.

Attack: 20 + Unable to parse dice roll.

Attack: 24 + Unable to parse dice roll.

Bonus action Attack: 16 + Unable to parse dice roll.

Tried to add bless in there but didn't work but normal rolls should be enough

1 11 damage

2 10 damage

3 13 damage