Hifu turns to Trek, Grek and Scorb and says in an incredibly enthusiastic way, in Dragon, “Once more into the breach, we must attack the brain creature, for our lives and for freedommmm!!  Attack now!”  He then switches to common and says “I’m coming Gilgi!  Hang on!”

Hifu steps into the room and chucks a javelin as hard as he can at the creature, then ducks back to stay at a 30 ft distance, using a stalagmite for partial cover.

Attack : 13.   Damage : 8

Nyx scuttles to the side to not block the way of other combatants and then sends an Eldrich Blast to the creature:

Attack: 7 Damage: 8

Kravek will run in as the noise of combat starts. If he has the room to move past Gilgin without triggering an attack of opportunity he will move around from that side and get between the creature and the water keeping some distance between them. 
If there is not enough room he will move through the water hanging on to the rock/pillar to the creatures right as Kravek moves through the water to get to the other side. 

Kravek will try to make himself as big as possible by holding his shield to his left and his right arm streched to the other side to discourage the creature from coming towards him. Should it still move towards Kravek he will try to hit it back with his shield. 

Movement: moving to get between the fissure and the creature. 
Action: Ready action if the creature tries to move to the fissure Kravek will Shove it with his shield trying to knock it back or out of the air. (Athletics 6 vs target Athletics or Acrobatics ,ouch a swing and a miss)

James will charge in after the others, trying to position himself so that the creature has less options for escape.

James swings his greatsword at the creature.

"Die foul creature!"

Gilgin, distracted by the pain of injuries, barely misses the beast with his swing, but Hifus javelin hits true. Nyxs blast goes wide and harmlessly explodes against the opposing wall. Kravek, you manage to get between the beast and the fissure, but when you step into the water, you feel the current is stronger then you expected, roll a DC 10 Strength check. 

Trex hurries into the cavern and tries to pull the bloodied Gilgin away from the monster. (If you want to resist roll Athletic or Acrobatic DC 1)

 But as he gets further away from the beast, one of its tentacles leashes out and tries to hit Trex (10ft reach):

Attack: 19 Damage: 12

But the Kobold dodges and keeps pulling Gilgin.

Scorb and Grek try to keep their distance, only peeking around the corners of the tunnel long enough to throw their javelins, one of them actually hits.

Attack: 2 Damage: 2

Attack: 2 Damage: 2

Kravek:

Once James posts his turn, Round 2 will start. 

(Kravek str check 14, also hanging on to the rock/pillar just in case :))

Kravek, you take 3 bludgeoning damage. On you turn make an Athletics check DC 10, on a success you manage to pull yourself against the current out of the fissure, on a fail you take another 1d6 damage and must succed a DC 10 Acrobatics check or ...

As Kravek enters the water he slips on a rock and falls down onto the side as he gets dragged in by the current. 

Trying to get back up he will (if the creature gets close) try to shove it back in the direction of the group. (assume the nat1 is fail for this) so he will have trouble keeping his balance with the strong current and only do random swing hitting nothing but air. 

If there is time, Hifu will either throw a rope to Kravek and brace it with a stalagmite to provide leverage and a handhold for him to get up, or if he was in range, lend my hand as a help action to pull Kravek up.

Gilgin swings again with the warhammer, still glowing white with heat.

Attack: 12 Damage: 8 plus 4 fire damage. 

He then grabs his shield to defend himself

Athletics check to resist the kobold. (Pretty sure it’s an auto succeed) 17

 Hifu comes back into the chamber and drops the torch before throwing one end of a rope into the creek and holding on to the rocks. (Krevek, make your athletics check for this round with advantage.)

Trex not able to pull away Gilgin swears: „You pinhead of a dwarf, that thing is dangerous !“

The monster tries to hit James with its tentacles .

Attack vs James: 5 Damage: 7

and with its beak it tries to finish of Gilgin

Attack vs Gilgin: 21 Damage: 8

distracted by Trex, Gilgin fails to block the monsters attack and it tears open his neck with its beak. The dwarf collapses to the ground badly bleeding.

Trex throws his Javelin, while Grek and Scorb attack it with their daggers.

Javelin Attack: 8 Damage: 2
Dagger Attack: 24 Damage: 8
Dagger Attack: 5 Damage: 3

Grek manages to plunge his dagger into the giant brain of the beast.

The monster looks very bad, but is still fighting wildly.

waiting for Kraveks save and Winenose before starting the next round. Also Gilgin to increase the tension, send me your death saves as a pm.

Kravek will attempt to pull himself back onto the solid ground. (Athletics save 9 and in case he can do it with advantage from Hifu's rope 6)

Kravek will pull himself out of the water using the rope Hifu provided and by standing up in the water, his large body providing enough mass to get himself some grip onto the ground and move out. 

(i assume this takes my full action?)

Yes it does, but you manage to safely climb out of the creek.

My movement is 30 ft.  I guess my action as well is helping Kravek get out of the water.  

Hmm? I must have missed a round going by, sorry.

Nyx will shoot another Eldritch Blast at it:

Attack: 15 Damage: 7

Hifu jumps up and shouts to James and Kravek - - "Attack the brain!"  He rushes over to where Gilgin fell, "Gilgin don't you die on me, hold on!  Don't you die because of me and my foolish plan."  Hifu puts his head on his lap, tears a few cloth strips from his clothing and tries to make a makeshift bandage around his neck to stop the bleeding.  

Hifu attempts to stabilize Gilgin.

Medicine check :  5

"I can't do it.  Trek, quick, give him mouth to mouth resuscitation!  Hang on Gilgi!"

James takes another swing at the brain in a desperate attempt to distract it from the fallen comrade.

Attack: 5 Damage: Unable to parse dice roll.

The beast moves it main body out of the way, but James still manages to cut of two of the tentacles (hit).

Round 3:

Quote from Twombley >>

Hifu jumps up and shouts to James and Kravek - - "Attack the brain!"  He rushes over to where Gilgin fell, "Gilgin don't you die on me, hold on!  Don't you die because of me and my foolish plan."  Hifu puts his head on his lap, tears a few cloth strips from his clothing and tries to make a makeshift bandage around his neck to stop the bleeding.  

Hifu attempts to stabilize Gilgin.

Medicine check :  5

"I can't do it.  Trek, quick, give him mouth to mouth resuscitation!  Hang on Gilgi!"

  
You are unable to stop the bleeding.

The monster looks really bad and attacks James who keeps chopping of pieces of it.

Attack: 24 Damage: 9

the monsters tentacles pierce James arm and wrap around his body, you see a green liquid oozing out of the wound and the spikes in the tentacle. James you are grappled, 16 and paralysed. 
You can‘t take any action or reaction this round, but roll another DC 11 con save to see if you overcome the poison at the end of your turn.

With one threat neutralised, it tries to take a bite out of Grek the kobold:

Attack: 14 Damage: 8

15 It bites one of the kobolds arms and he screams in pain, but he is  still standing

Gilgin’s arm jolts up and grabs Hifu by the scruff of the neck as he crouches over Gilgin. Gilgin sprays Hifu with spittle as he barks “Over my dead body!”