(100% here)

(I'm still at the same health that I was at, but not been hit yet of course.)

(Barosh is still at full, he took a hit from the Solar earlier but healed over the short rest)

Con Saves vs. Stunning strikes: 14 22 14

WELL... it really does not want to be stunned and that is going to use up its last two legendary resistances!

Raze you dash in and pummel the Beast. Jamming your staff into weak spots between scales.

That bring us to Hornet and Archy. The monster is right in front of you. Mankur is on your left and Gilra's dome is on your right.

Hornet helps Archy attack as Archy yells out from his blast hitting himself, cursing in abyssal. He attacks twice, then takes a leap and hopes the tarrasque is affected by poison.
Claws: Attack: Attack: 24 Damage: 7 and Attack: 15 Damage: 7
Hornet's poison:Attack: 6 Damage: 3 and DC 11 con save or be poisoned. 

[ah, crap.]

Archy: Unfortunately your claws and Hornet's stinger glance off of the tough scales and don't damage the creature.

That brings us to Ileanys. Having used most of your movement to back up last turn, you are now about 65ft from the Tarrasque. You also have inspiration from Laucian should you need it.

(Isn't it literally impossible for the Tarrasque to fail a DC 11 con save?)

(Depends on if the DM considers nat 1 on a ST a crit fail...)

(I would call it a crit fail. Then he would be paralyzed, and couldn't attack... Eh, it was a long shot.)

Ileanys fires 3 more arrows at the beast, then flies straight up with his 60 foot movement speed.

Attack: 24 Damage: 30

Thirsting Blade Attack: 28 Damage: 32

Crossbow expert bonus Attack: 29 Damage: 35

 Edit: Use the bardic inspiration on the 24 to add 9

Barosh will cast a 7th level, quickened disintegrate at the beast and then cast frostbite on it as well.

DC 19 Dex save or 96 force damage

DC 19 Con save or 9 cold damage and disadvantage on its next weapon attack.

Then Barosh will fly so that he is 75 ft above the Tarrasque.

Ileanys you quickly fire three crossbow bolts, all of them finding their mark.

Dex save: 18  The creature roars as the spell hits, bits of its hide flaking off into dust.

Con save: 20 The frostbite spell however has not effect. 

Barosh: You fire your spells, and though your hands tremble from the creature's terrifying presence, you deal significant damage to the Beast. You fly up to the cavern's ceiling.

(The cavern is 100ft high and the Tarrasque is 50ft tall so the highest you can get is 50ft above it. You are currently frightened and cannot get closer. Now that it's the end of your turn, make a DC 17 Wisdom save, on a success you are no longer frightened.)

That brings us to Gilra. The massive creature is at the edge of your anti-life shell, 10ft away from you.

Wis save: 10

Gilra is relieved that the spell worked against this creature. For added insurance she is going to use Symbiotic Entity once more for temp HP (and other benefits) and then throw a handful of spores forward towards the centre of the creature  

(Spreading Spores: 

While your Symbiotic Entity feature is active, you can hurl spores up to 30 feet away, where they swirl in a 10-foot cube for 1 minute. The spores disappear early if you use this feature again, if you dismiss them as a bonus action, or if your Symbiotic Entity feature is no longer active.

Whenever a creature moves into the cube or starts its turn there, that creature takes your Halo of Spores damage, unless the creature succeeds on a Constitution saving throw (DC 20). A creature can take this damage no more than once per turn.)

So at the start of its turn it will take 2d10 necrotic damage (2 because symbiotic entity being active doubles the default 1) unless it succeeds on the CON save of 20.

And Gilra will stay where she is. 

Gilra: You cast your spores forward into the Tarrasque's space. 

That brings us to the Tarrasque's turn.

Con save VS Spores: 23 which is a fail, so it take 16 necrotic damage.

It will use Frightful Presence. Laucian, since you are no longer in Mankur's aura I need a DC17 Wis save from you or you are frightened.

For the Bite-  Gilra (1), Bathazorin (2), Archy (3), and Mankur (4) are in range. 3  It's jaws clamp down on the barrier protecting Gilra, unable to pass through. 

Int Check: 13 Nope it hasn't given up on trying to get the druid yet.

1st Claw - Gilra (1), Bathazorin (2), Archy (3), and Mankur (4) are in range. 4

Claw vs Bathazorin: 29 for 40 non magical slashing damage

2nd claw- Gilra (1), Bathazorin (2), Archy (3), and Mankur (4) are in range. 1

BONG the claw scrapes the barrier.

Horns - Gilra (1), Bathazorin (2), Archy (3), and Mankur (4) are in range. 4

Horn attack vs Archy: 38 dealing 22 non- magical piercing damage

Tail - has a range of 20ft so Gilra (1), Bathazorin (2), Archy (3), and Mankur (4) AND Raze (5) are in range. 4

Tail vs. Raze 33 for 25 non magical bludgeoning damage and I'll need you to make a DC 20 Strength saving throw. On a failure Raze is knocked prone.

At the end of its turn, the Tarrasque regains its legendary actions. 

That brings us to Mankur. The Tarrasque is right in front of you.

Attack at disadvantage please from Cloak of Displacement. If still hits, here's the save.

StrST 27

Quote from Saute_Monkey >>

Attack at disadvantage please from Cloak of Displacement. If still hits, here's the save.

StrST 27

 

 Not finding a clear ruling on this- but the Tarrasque has blindsight and I'm inclined to say it's not effected by the visual illusion created by your cloak. Let me see if I can find anything official.

 Well, shoot! Forgot it was an illusion that granted the disadvantage. You're ruling makes sense to me. The description specifically calls out it being an illusion.

Wis save: 18

Mankur prepares a staggering smite before making 2 attacks with his long sword

First attack

To hit: 19 Damage 21 Smite damage 13

(Need a WIS st DC 19 if it hits)

Second attack

To Hit 21 Damage 22