Con save: 22

Yusko stuck at it.  He wasn't going to go anywhere, at least not until this Mermaid thing was dead.  Digging down into his reserves he struck out with the knife twice, his fist once and then hit some pressure points on himself to stop his own bleeding.

Action: 2 attacks

Bonus action: last flurry of blows point spent, able to use one healing hand on himself to gain 4 HP

Attack: 20 Damage: 10

Attack: 16 Damage: 11

Attack: 25 Damage: 5

Yusko had probably awakened something in the mermaid - a curse of many underwater predators who lose their mind at the smell of blood. She did not run away but engaged in the simple mele like the simplest of sahuagin would - mindlessly and brutally.

claw vs Yusko: Attack: 26 Damage: 10

Trudie and Nameless!

After a few slow and steady blinks, Trudie lifts a finger.

(Is this where my misplaced #507 post for slow should go now? Lol I’m fine with carrying that action over to free Hansel.)

Having left off the hopeless struggle to escape from the rock, Nameless shoots at the mermaid, hoping that if they kill it she'll be able to win her freedom....

Attack: 11 Damage: 10

Round 8 (continues)

Hansel did not get back his freedom yet, but his head was clear now. As clear as the water around the battle, since the dark cloud dispersed.

However, the tentacled creature was not gone. Was not even harmed yet and fully intended to keep Nameless girl preoccupied while her master was fighting Yusko:

vs Nameless: Attack: 14 Damage: 24

Vadok's roll 15

Somehow, the tentacled woman managed to miss completely immobilized Nameless girl. But the mermaid was not about to miss her prey. She attacked viciously:

vs Yusko: Attack: 20 Damage: 10
                  Attack: 30 Damage: 10

Map:

  Davis, Yousko, Hansel! 

Vadok tried his best to send another healing wave: 13 HP (18 ) but this time the help was moving too slow and Yusko was way too heavily injured.

(Davis, all attackers are at 50-55 ft)

Davis again attempts to break free the nameless girl...

Athletics: 24

The battle has moved almost out of Hansel's reach, and Yusko is too far for some of his spells. Still, the cleric blows a kiss toward Yusko, hoping it will give him enough of a lift to continue fighting, while he exerts his strength in attempting to free himself from the grasping coral.

Bonus Action: Healing Word at 2nd level on Yusko (range 60') for 12 HP.

Action: Athletics check to break the restrained: 15

If Hansel gets free, he will move 30' to about #AD7.

Technically Davis goes before Hansel and Hansel is closer to Davis, so, combined with "again" it would make sense that he tried to free Hansel. If so - Hansel is free for his turn and can take an action.

(works for me.)

So from my understanding, Yusko went down, would have failed a death saving through due to the 2nd attack, require a saving through: 18 results in 1 save and 1 failure.  Then get healed 12 points by Hansel.  Is that correct?

No, it's a retconed math. Yusko had 22HP, got hit for 20 and now is at 2 HP but still can act. (Whether or not Hansel uses Healing Word, now that he is free and can choose something else, is up to Hansel)

With one hand, Yusko reaches into the sack at his side and pulls out a potion and drinks it down.  With his other, he stabs at the mermaid.

Bonus action potion of greater healing: 14 HP gain

Action Attack:

Attack: 13 Damage: 9

Attack: 19 Damage: 8

Hansel will instead:

- Move to #AE13 or thereabouts (within 30' of all friendlies)

- Use his action to cast Beacon of Hope on all friendlies. There is no healing in there, but it will amplify all healing, including Trudie's if she does that next.

- Use his bonus action to move the Spiritual Weapon 30' closer to the enemies.

(Yousko gets 20HP for BoH)

The enemy did not fall, but neither did the heroes, to the immeasurable frustration of the former. The mermaid almost screamed when the practically dead monk managed to quaff a potion and colours returned to his face.

Round 9

The battle was still happening in that one spot - with tentacled creature searching for the vulnerable spot in Nameless' girl defences and the mermaid locked in the vicious melee. The mermaid used her caws again:

claw vs Yusko: Attack: 19 Damage: 10

Math

Trudie and Nameless!

Nameless girl continues her single minded attempts to slay the mermaid fighting with Yusko. 

Attack: 20 Damage: 7

Attack: 29 Damage: 6

+ 5 extra damage on a hit.

Trudie would twist around as if she were looking for something. "Here Squishie... Squishie... Squishie..." The old tort calls out for her spirit as she feels around with her staff. Eventually the mini jellyfish illuminates at Yusko's feet.

On Yusko
Casting: Healing Spirit lvl 4 Healing: 3d6+6 = 24hp with beacon of hope
(Concentration on Healing Spirit: 4/5 uses)

This time both bolts hit and hit bad enough for the mermaid to remember, she had more than one enemy around. She looked at the approaching group, back at Yusko, and then - pointedly - at the tentacled creature. The woman jerked and in turn hissed at the Nameless girl. But even before she could attack, another wave of undead raised from the floor, forming a wall between the fighting two and the recovered group.

The mermaid kept Yusko for herself but sent one undead to help bring down the Nameless girl. The rusty sword was about to strike, but not before the tentacled creature tried to kill the girl herself!

vs Nameless Attack: 14 Damage: 40 thunder damage

Whatever the thing was trying to achieve was washed off with Vadok's healing magic, which finally reached the others and was greatly multiplied by Hansel's (28 HP). For Yusko that heal and Trudie's also affected healing came right in time, because the mermaid and her undead minions attacked in a unison!

M1 vs Nameless:  Attack: 12 Damage: 6
                                 Attack: 23 Damage: 7

Mermaid vs Yusko: Attack: 10 Damage: 9
                                   Attack: 17 Damage: 9

M2 vs Trudie:  Attack: 20 Damage: 4
                          Attack: 26 Damage: 10

M3 vs Davis:  Attack: 26 Damage: 5
                         Attack: 13 Damage: 7

M4 vs Davis:  Attack: 14 Damage: 5
                         Attack: 18 Damage: 8

M5 vs Hansel:  Attack: 16 Damage: 4
                          Attack: 22 Damage: 6

On hit from undead - DC 12 Constitution saving throw!

Math (previous health + 28HP - new damage)

Map

 Davis, Yousko, Hansel! 

Nameless one is at 26 hps after taking 43 damage that round.  Con save vs DC 12 : 18