(Idk how to do spoilers on mobile. Idk if I can. But I might be away for a while and only jarl is before me)

Celric casts spiritual weapon 60 feet away at C14 taking the form of a warhammer with the crest of the platinum dragon on the sides.

attack: 26 damage:8

Celric then dashes 60ft north to C13 and stares mind flayer right in the eye as spirits swirl around them both. 

spirit guardians wisdom save 15, damage:19, half on successful save. Takes damage again at start of their turn.

(also thanks I totally forgot about death ward)

Jarl moves to Q22 and casts Moonbeam at 3rd level. DC 16 CON save, 20 radiant damage, half on save.

For his turn Flynt is going to dash - jump out of the water and make a mad dash for the next pool and jump into it. 

Readying an arrow, Balian edges out from the pillar's safety and watches the form of the mindflayer distort and swim in the air.  Frowning, Balian recalls the time he entered an archery competition blind-drunk, and tries to relax his eyes, not concentrating on the creature, instead looking for where the various images are on average, before loosing his arrow towards the target, and returning to his hiding spot.

Mirror Image Targeting Roll: 6

Whether by skill or luck, Balian's arrow strikes the solid body of the mind-flayer, embedding into its flesh in a way that should do some serious damage... But somehow it doesn't seem to do any; the mind-flayer simply snarls and the arrow pops back out and falls to the ground, the hole closing instantly (it seems to be impervious to this type of damage)

Balian: Since you attacked, your position is revealed unless you hide again.

Shale charges forward into the middle of the chamber near Celric (O12). Once again he calls upon his goddess to mend the wounds of all around him.

(healing 10 for all; additional 5 for Shale. Note: Balian is not in your line of sight at O12, but you could have cast on the way there so we'll say it affects everyone)

The cleric then begins the preparations of a spell should the enemy come in his range.

(Ready Toll the Dead if it comes within 60', DC 16 WisST, 17 (uninjured) or 17 (injured) on fail)

Jarl moves to Q22 and casts Moonbeam at 3rd level.

(Mind-flayer CON save: 17)

 The creature seems to have an uncanny ability to resist the effect (advantage on saves vs magic), but is nonetheless singed by the radiant energy (10 radiant damage).

Celric casts spiritual weapon 60 feet away at C14 taking the form of a warhammer with the crest of the platinum dragon on the sides.

However, the mind-flayer seems to easily and casually side-step the hammer's blow.

Celric then dashes 60ft north to C13 and stares mind flayer right in the eye as spirits swirl around him, just below the mind-flayer hovering 30' in the air. 

WIS save: 26

The mind-flayer again seems to be able to resist the worst of the effect, but is again lightly charred with the radiant energy (10 damage).

Note: Spirit Guardians doesn't do damage until the start of the creature's turn, but it's easier to keep track of to do it here.

Having been burned twice in rapid succession, the creature lashes out at Celric with a tentacle (legendary action). Attack: 19 Damage: 22, but doesn't manage to connect. (Edit: Oops, it's up in the air so it can't do that)

 The mind-flayer points a finger at Jarl, retaliating against the moonbeam by hurling three more darts of force energy (legendary action: 11 force damage to Jarl)

Brix runs east (M12) to get a clear shot at the creature assaulting the group, raises his crossbow and fires two bolts.

mirror image roll 1: 18

Attack 1: 27 Damage: 7

mirror image roll 2: 12

Attack 2: 23 Damage: 5

 The first bolt hits one of the illusory duplicates, causing it to vanish. The second bolt does the same with another one of the images.

The mind flayer launches one more volley of magic missiles, this time at Celric (legendary action: 12 force damage)

Flynt jumps out of the water and makes a mad dash for the next pool, jumping into it (in the water at L9; note, fyi, that at combat pace you can be out of the water, since you can hold your breath for up to 2 minutes, or 20 rounds).

Squiggles moves closer (T11) and launches a bolt of fire at the Mind Flayer, hitting it squarely in the torso. Unlike the previous attacks, it does not appear to resist the effect, bearing the full brunt of the magical flame (14 damage).

The mind-flayer floats gently forward and down, landing on the ground between the two northern pools of water (H13), and passing through Celric's spirit guardians (WIS save: 18), seemingly unconcerned, though it does seem to get singed a bit on the way (10 damage). To Jarl and Celric it just seems to stand there doing nothing. Flynt, Brix, Balian, Shale and Squiggles, however, feel an intense assault on their minds.

 Squiggles, with his highly developed intellect and natural gnomish resistance to magic, is able to shake off the effects. And somehow, the brutish Brix manages to resist as well. Balian, Shale, and Flynt, however, are not so fortunate, as the psychic blast tears at their minds (33 psychic damage), leaving them stunned (you can each make another INT save at the end of your turn to snap out of the stunned condition; DC 18).

(Ooc: just asking for clarification

”when the creature enters the area for the first time on a turn or starts its turn there, it must make a Wisdom saving throw.”

it says enter for the first time does that mean if it willing enters? Because I was reading it like as soon as it’s in the zone it takes damage and once more at the start of their turn.)

"Balian!" Jarl calls. "Ammo!" (if Balian comes to Jarl, and he ought to make it all or most of the way to Jarl if he uses everything for movement, Jarl can cast Magic Stone to give Balian something that will do some damage, though not sneak attack). (Cleared this with Esker as something that they would have probably done in the past working together.)

Balian (stunned)
Shale (stunned)
Jarl (concentrating on moonbeam; needs a DC 10 CON save to maintain)
Celric (concentrating on spirit guardians; needs a DC 10 CON save to maintain)
Brix (raging, 12 HP)
Flynt (stunned)
Squiggles (bein' Squiggles)
Mind Flayer (-34; mirror image: 1 image remaining)

Edit: actually I guess if Celric loses concentration it would have happened before the mind-flayer moved, so it would only be down 24 instead of 34

Just remembered Shale's held toll the dead, which would have gone off before the mind blast: WIS save 23

No effect

IntST to break stun 10

(Whoa! Double digits. We're getting there.)

Balian

INT save: 5

After being cursed by the trap, Balian's mind is further assaulted by the mindflayer, and he begins to drool, responding to Jarl with a "Huh?"

Con save: 13

Balian: uncanny dodge unfortunately only works on attacks, not saving throw effects 

Quote from Esker >>

Balian: uncanny dodge unfortunately only works on attacks, not saving throw effects 

 OOC - Dang, worth a try! PS, I added a stealth check to hide to my previous post.

INT Save - 13

CON save: 9

Jarl loses concentration on Moonbeam, cursing. Gathering his magic to himself, he casts Tidal Wave, sending water from the pool on the right crashing over the mind flayer and into the pool on the left. DC 16 DEX save, 10 bludgeoning damage plus knocked prone, half and not prone on a save. 

He then moves to M23, trying to get some cover behind the pillar, and casts Magic Stone on three pebbles, which can be used by any party members.

Celric and Squiggles left to take turns this round (this is more a reminder for me than for you)

Neither Balian nor Shale are able to snap themselves out of their stunned stupor.

Jarl loses concentration on Moonbeam, cursing. Gathering his magic to himself, he casts Tidal Wave, sending water from the pool on the right crashing over the mind flayer and into the pool on the left, then moves to M23, trying to get some cover behind the pillar, and casts Magic Stone on three pebbles, which can be used by any party members.

DEX save: 12

The mind-flayer is unable to maintain its balance in the face of the rushing water, and falls prone, getting a bit battered in the process (10 damage). Anybody holding a torch within 30'? I guess I'm assuming those without darkvision had light cast on something on their person at some point, so no actual flames or hands occupied.

(Ooc reminder very welcome.)

As a bonus action Celric commands his spiritual weapon to move south 20 feet and attack. 23

Damage 4

The Celric will move directly south close enough to engulf his foe in his spirit guardians again but far enough away not to hit himself with his next attack. 

“Bahamut, rain fire on my foe!”

flame strike

Fire 19

Radiant 10

DC dex 15 or half

Spirit guardians 19

Celric, you can roll your spiritual weapon attack with advantage since he is prone. You'll hit regardless, but you might crit.

DEX save vs flame strike: 19

(2+14 damage; cumulative damage total 60)