Troubles in Paradise

Before I add updated math and map - please, everyone (but Yusko) make your saves and count damage.

Trudie Dex ST (vs 13): 18 

One of the runes on Davis's chest flares bright as he activates it. The second attack from the invisible foes is redirected, the damage going to the mermaid herself...
Attack: 26 Damage: 18 is transfered to the mermaid.

Otherwise Davis is updated with damage. He is now at 50 / 75 HP.

Hansel DEX save: 16

The mermaid jerked, not expecting to be hit by anything and yet getting a long scratch as if from a sharp tusk. The gaze of the creature shifted to Davis.

Math

Map

 Yusko, Davis, Hansel!

Davis knows that there are two seals nearby (and attacking him personally). He can also see (and get to at the cost of 2 AoOs) the mermaid and her helping hand - the tentacled woman. (All undead are running away and mostly can not be reached by now) 

Nameless Girl DC 18 Wis save : 19

Hansel moves away from the others, hoping to spread the group out a bit and limit the damage of any further multi-target or area attacks. (Say about 30' back or to either side, whichever is safer).

He then directs the spiritual weapon to float over to the mermaid and try to give her a spanking (it can move 20' from current location. If it's too far, then just attack closest target.

Attack: 21 Damage: 6 force

Finally, he then casts Guiding Bolt (first level) at the mermaid, trying to give her that magical glow that all lovely ladies crave...

Attack: 26 Damage: 13 radiant and...

... if that hit, then the next attack roll made against this target before the end of Hansel's next turn has advantage, thanks to the mystical dim light glittering on the target until then. I think Trudie is next, but if she doesn't attack the mermaid, then perhaps the advantage would be good for Nameless to get Sneak Attack.

The weapon swam towards the enemy not quite reaching but the bolt hit (lighting up the disproportional grotesque figure), immediately attracting the mermaid's attention to the young cleric. The crystal on top of the rod was now pointing at him, and with the flash of light sent a charge of lightning toward Hansel.

Attack: 29 Damage: 9

Attacks from Yusko and Davis are coming with advantage.

 Yusko, Davis!

Davis attempts to attack one of the invisible foes nearest him...
Attack: 16 Damage: 15
Attack: 14 Damage: 16
Action surge...
Attack: 14 Damage: 17
Attack: 12 Damage: 16
Bonus action polearm attack...
Attack: 21 Damage: 11

The first of the above that hits does an extra 1 damage from giant's might and 4 damage from his patron.

The first attack that hit also does an extra 7 fire damage and requires a DC 14 strength save...
Fire Rune (Special)
When you hit a creature with an attack using a weapon, you can invoke the rune to summon fiery shackles: the target takes an extra 2d6 fire damage, and it must succeed on a STR saving throw (DC:14) or be restrained for 1 minute. While restrained by the shackles, the target takes 2d6 fire damage at the start of each of its turns. The target can repeat the saving throw at the end of each of its turns, banishing the shackles on a success. Once you invoke this rune, you can’t do so again until you finish a short or long rest.

It looked as if Davis developed the additional senses inhabitants of these places had - again and again the blade of his glaive hit something soft and fleshy and droplets of dark blood formed the cloud around the battle-spot, quickly dissolving in dark water.

The animals were still there, the warrior was sure of it, but he also knew he wounded both of them.

At this point, the mermaid glanced once again at the stubborn girl that remained paralyzed, but instead of expected attack, moved back between the central columns of corals.

Yusko!

(Save for firey shackels? or is the fire non-magical so not useful under water?)

Still processing - fire effects are supposed to be suppressed underwater. On the other hand, if electricity is unaffected... I say the shackle effect works (Save for the first seal: 9 ) and we go with "super hot" and rests to it. 

First seal is restrained now.

"I'm gonna go try and hold the big girl still.  Hurry on after..." Yusko says and pushes off the sea floor.  With bizzare speed he cuts through the water like it isn't even there.  Moving though in a slight arc, he cuts his way around the two between them and the mermaid and shoots towards her.  With knife in hand lashes out twice at her.

Move and bonus action dash. to move to Mermaid B.  Going to skirt a little north of M3 and T to avoid attacks 

Action: 2 attacks against B

Attack: 27 Damage: 11

Attack: 29 Damage: 13

If one of those hits, add 3 for sneak attack damage.  If the first hits spend a ki point for stunning strike DC 14 con save or its stunned,  if it saves and then 2nd strike hits then spend another ki point to stun it.

Nice! double that sneak for a total of +10

B Con save (DC14): 1. 18 2. 11 

Up close the horrible creature did not look any better, but the important part was the creature could be hit, hence, killed. Something Yusko proved with his fast attacks - both strikes hit, both left the mark. It even looked like the creature was stunned by the barrage of blows, but then she shook it off, still glaring and very much moving. The mermaid did not leave, though, and remained a target for the sharp eyes of the group.

Round 3

Math

Map

 Trudie and Nameless One!

Nameless one finally sees their enemy revealed.  She moves to a better position and unleashes a couple bolts. 

Move:  AE-11

Bonus Action: Move Hunters Mark to B.

Action: Attack action.

Attack: 19 Damage: 7 + 2 Hunter's Mark

Attack: 16 Damage: 5 + 5 Hunter's Mark

Trudie would mosey over a little so that the group of them aren’t clumped up together. Then once again she’d take position and rear up her staff. Instead of batting off, this time it looked like she casted an imaginary fishing line. A snag? Remembering to flex her old nan muscles, she’d yank. Hard. “Oopalai!” With a grunt, a wall of thorny coral bursts from the sea floor.

Action: Wall of Thorns Damage: 28 Dex DC16 Placement: 60ft diagonal B through to M3 without snagging Yusko 

Saves: 9 and 2 (M3 is dead)

The Nameless girl took the aim and sent the bolt into the bloated body of the mermaid. The creature answered, though, not to the girl. Ruby-red claws tried to slice Yusko:

Attack: 17 Damage: 9 

It was preparing another attack when the fast-growing mass of thorns sprang between the corpses, ornamenting the floor, and tried to envelop the mermaid and her tentacled helper. M3 was sliced to peaces before it even attempted to crawl away, and the sharp needles pierced the flesh of both sea monsters. 

The mermaid swam up and behind the column (no AoO!) but not before more undead raised around the adventurers. 

The still invisible seals continued their attempt to eat up Davis, though one, bound by the hot shackles, lost its advantages over the prey) :

Attack: 18 Damage: 11
Attack: 18 Damage: 14
Attack: 22 Damage: 17
Attack: 27 Damage: 22

The tentacled creature (or what was left of her) pulled herself from the wall, moving towards the party as if going to ram into someone. Instead, at the last moment, she disappeared with the thunderous sound (Nameless girl thought she saw some movement in the ship to the sought.

Trudie, Hansel and Davis:  16 thunder damage or half on Con save DC13 

"No-no, that should not do!" Vadok raised his hand and with the most kind smile shad the vitalizing green light on Davis: 8 HP 

This is when the new downers attacked: (One a hit - make  DC 12 Constitution saving throw!)

vs Nameless:  Attack: 9 Damage: 6
                          Attack: 24 Damage: 5
                          Attack: 9 Damage: 5
                          Attack: 23 Damage: 5

vs Yusko:  Attack: 14 Damage: 10
                   Attack: 8 Damage: 4

vs Trudie  Attack: 16 Damage: 5
                  Attack: 12 Damage: 8

vs Hansel  Attack: 19 Damage: 7
                   Attack: 21 Damage: 7

 4

There was no attack from the mermaid, only silent undead (now injured by the tentacled woman) continued swinging their swords at the group. 

(Seal save: 7 )

Yusko, Davis and Hansel! 

(Seals keep attacking Davis and both are free now, undead - others, and bigger bosses retreated and hanging somewhere in the dark)

The seal in the shackles takes 7 fire damage at the start of its turn. It gets a new DC 14 str save at the end of its turn.

davis con save: 27

Trudie Con Save vs DC13: 23 (Full Thunder Damage)