Omen will cast Sacred Flame on the creature. Dex save DC 14

Damage (if fails save): 8

Omen will move out of melee range of the creature. 

Jimena's Turn: Round 4

Jimena drops her warhammer, and aims the hand-crossbow at the Tendriculos so that she can engage in a ranged combat. The weapon releases a single bolt when the trigger is pulled.

Action: Attack [Hand Cross-bow] x Tendriculos: Attack: 11 Damage: 6

(Tendriculos' Dex Save: 0) The tendriculos seems completely unprepared and is hit hard by Sir Omen's Sacred Flame.

Sir Jimena's bolt also lands and deals damage. 

Sir Glint makes a Constitution Saving Throw 15

The tendriculos makes a perception check (1; That's two natural 1's in one post). It cannot tell which direction the bolt came from. It moves in a random direction:

1       2      3
4       X      5
6       7      8
(2 is directly towards the party, 7 is directly away from the party, on a 9 or 10 it does not move)

5; The monster moves perpendicular to the party.

Omen will pull out a mica chip from his belt and chant small incantation, casting Shatter. Omen will position the spell as close as he can without hitting the party, besides the swallowed Sir Glint.

Con save of 14

Damage if fails: 9 thunder damage

(@Wizardously, Can Jimena discern the condition of the Tendriculos? Is it responding to the magical damage in a different way?)

(It is still healing from the force damage from eldritch blast and the thunder damage from shatter in the same way as the melee damage)

Jimena's Turn: Round 5

Jimena drops her shield so that she can use the free hand for loading ammunition. Then she will pull the trigger to release a single ammunition from the hand-crossbow while moving 20 ft forward.

Special Action: Action Surge: Doff the shield

Action: Attack [Hand Cross-bow] x Tendriculos: Attack: 8 Damage: 3

(Tendriculos' Con Save: 4) The monster is again completely unprepared for Sir Omen's spellcasting, this time Shatter.

Sir Jimena's bolt goes wide.

Sir Glint makes a Constitution Saving Throw 20

The tendriculos makes a perception check (8). It cannot tell which direction PC's are. It moves in a random direction:

1       2      3
4       X      5
6       7      8
(2 is directly towards the party, 7 is directly away from the party, on a 9 or 10 it does not move)

7; It moves 15 feet away from the party. It's currently 50 feet away, and can no longer be seen by the PCs due to the mist obscuring it.

Two more bolts of golden energy fly from Omen's eyes striking at the creature.(Omen casts eldritch Blast )

Bolt 1 to hit: 20
Bolt 1 damage, if hits: 10

Bolt 2 to hit:13
Bolt 2 damage, if hits: 8

Omen will move to the right of the creature 30 ft.

"Sir Omen, we need to hold the fire till we can locate the exact position of the creature"- Jimena will propose while moving 30 ft forward in order to actively search for the Tendriculos.

"We can still save Sir Glint if he's alive."

Wisdom: Perception 14

Additional Value (Knowledge from Past Life) 6

Omen nods. The inexperienced knight says:
" I had hope in hitting it"

With that Omen will take out a gold coin, take a small bit of phosphorescent moss and using the component, Omen will cast Light on the coin and toss it about 5 feet in front of his position.
(on his turn)

Sir Omen and Sir Jimena move forward and see the monster. It does not seem to move very quickly and instead preferring to attack from ambush.

(Assuming I go next)]

Golden/Red energy flies from Omen's eyes towards the creature
Eldritch Blast

1st blast: 8
damage, if hits: 5

2nd blast: 16
Damage, if hits: 8

Jimena's Turn: Round 7

Jimena will close the gap between the Tendriculos and the Psi Warrior to move within the melee range of the creature (15 ft). She slashes at the core of the opponent twice using the dimly lit Moon Touched Sword while holding the grip with both of her hands to provide extra force. Immediately after attacking, she moves backwards without paying any heed to the hazardous result of her movement (15 ft).

Action: Attack [Longsword] x Tendriculos: 

• Attack: 8   Damage: 12 
• Attack: 17 Damage: 12

(Rolled in gamelog)

Sir Omen's rays strike true, though the second seems to have very little effect, glancing off a tentacle as the monster turns towards them. As a result of its turn, Sir Jimena's first attack likewise misses her aim. But she adjusts and her second swing strikes true.

The monster makes an opportunity attack against her as she recklessly retreats.
Tentacle Opportunity Attack: 27 Damage: 20

The monster, while lacking intelligence, can follow Sir Jimena's movement and moves towards her to attack.
Tentacle 1 Attack: 17 Damage: 14
Tentacle 2 Attack: 26 Damage: 11

The attack of opportunity punishes Sir Jimena for her brazenness, but, prepared, she blocks the other attacks easily.

Jimena's Reaction: Protective Field

Jimena utilizes her psionic ability to conjure a temporary shield of telekinetic force that absorbs the damage partially. The reaction expends one psionic energy die.

Reduced Damage: 2

Omen will fire another set of Eldritch Blast at the creature.

First Blast to hit: 18
First Blast damage if hits: 7

2nd blast to hit: 26
2nd blast damage, if hits: 3

Jimena's Turn: Round 8

Jimena will disengage and move 30 ft backwards.