Blossom growls and approaches, striking the monster several times in whatever area he feels it is best supported by vines and roots, so as not to free it.

Attack 1: 13
Attack 2: 19
Attack 3: 10

Damage 1: 6
Damage 2: 5
Damage 3: 7

(Flurry of Blows)

The undead takes the blunt of the damage. It retaliates against the tabaxi with a trunk sized fist.
Attack: 8 Damage: 18

But the slow creature is easy for the monk to dodge.

Initiative order:
21 Thariv
18 Ufeza
17 Zephyra
17 Monstrous undead (26, 20 feet in front of the group)
14 Blossom
11 Luther

Luther goes next.

Luther draws his bow, dark tendrils snaking around the arrow's shaft. He lines up his shot, then shouts "Kee-ah-ree!" as he fires.

Longbow Attack: 20 Damage: 10 piercing

If the arrow hits, Luther will use Shadow Arrow - the undead takes 4 psychic damage and must pass a DC12 WIS save or be unable to see anything farther than 5ft away until the start of Luther's next turn.

Wisdom save: 7

Luther's arrow burries itself deeply in the undead, a puff of dark spores burst from its rotting flesh.

Initiative order:
21 Thariv
18 Ufeza (in melee with the undead)
17 Zephyra
17 Monstrous undead (40, blinded)
14 Blossom
11 Luther

Thariv goes next.

Thariv releases another blast at the undead monstrosity, holding off on conjuring the tentacle of the deep as the creature seems to be reeling.

Eldritch Blast Attack: 7 Damage: 5 Force Damage

The blast hits the undead. It's starting to show a few signs of damage, but pretty much still in one piece.

Initiative order:
21 Thariv
18 Ufeza
17 Zephyra
17 Monstrous undead (52, blinded over 5 ft)
14 Blossom (in melee with the undead)
11 Luther

Ufeza goes next.

Ufeza draws his light crossbow and fires at the undead.

Attack: 23 Damage: 12

The bolt wasily finds the undead, embedding itself completely in the necrotic flesh.

Initiative order:
21 Thariv
18 Ufeza
17 Zephyra
17 Monstrous undead (64, blinded over 5 ft)
14 Blossom (in melee with the undead)
11 Luther

Zephyra goes next, then the undead.

Luther sees his arrow strike the monstrous undead creature and the tendrils of shadow wrap around its head.  It is the first time that he has seen the effect, an arcane conjuration that he has been researching for so long.  He hopes that obscuring the creature's sight will give his companions an edge in the fight, and considers whether to try a different enchantment next or simply pepper the monster with more arrows.

Wolf-Zephyra hesitates only slightly seeing the burst of spores from the creature leaping forward at its base near her party-mate.

Bite (with Adv due to Pack Tactics) Attack: 16 Damage: 5   

Using her movement only to keep active due to try to avoid being potentially grabbed by vines.  

Zephyra bites deep into the undead, causing the vines to twirl and lash out. They try to grab Luther (attack roll: 15) and successfully pull him forward next to the giant undead.

The great zombie swings again against Blossom.
Attack: 19 Damage: 6

Initiative order:
21 Thariv
18 Ufeza
17 Zephyra (in melee with the undead)
17 Monstrous undead (73, blinded over 5 ft)
14 Blossom (6, in melee with the undead)
11 Luther

Blossom goes next.

On Luther's turn he fires again, a mundane arrow this time.

Attack: 18 Damage: 9 

And use Action Surge to fire again:

Attack: 17 Damage: 4 

Blossom grunts as he absorbs the blow, then carries on with his assault.

Attack 1: 25
Attack 2: 22
Attack 3: 21

Damage 1: 5
Damage 2: 7
Damage 3: 7

(Flurry of Blows)

The undead tanks the damage, flailing its vines around. It's noticeable damaged at this point. Some of the vines around the cave wall detach from the root system and crawl towards the huge undead, trying to constrict Blossom on its way.
Attack: 14 Damage: 8

Initiative order:
21 Thariv
18 Ufeza
17 Zephyra (in melee with the undead)
17 Monstrous undead (105, blinded over 5 ft)
14 Blossom (6, in melee with the undead)
11 Luther
  5 Tangle of vines

Thariv goes next.

(The undead will no longer be blinded; the effect ended at the start of Luther's subsequent turn)

On her turn, Zephyra-wolf charges again at the Undead Creature staying close to her companions.  

Attack: 12 Damage: 9  

(Pack Tactics for advantage)

Ufeza surveys the situation and takes action

If Thariv has dropped the Monstrous Undead to 0 hit points AND it has undead fortitude, Ufeza will cast Viscious Mockery as an action. Wisdom save DC 14 or take 2 psychic damage and have disadvantage on the next attack roll it makes before the end of its next turn. Ufeza will then use a bonus action for Unsettling Words. The creature will subtract 5 from its next saving throw before the start of my next turn.

If the Monstrous Undead remains above 0 hit points, Ufeza will reload and fire his light crossbow: Attack: 17 Damage: 5

Thariv clenches his coral talisman tight in his hand and directs another blast of mystic force at the undead horror. 

Eldritch Blast Attack: 19 Damage: 13

Thariv's blast hits the undead (undead fortitude roll: 16) and it collapses motionless onto itself.

Ufeza quickly aims his crossbow at the tangle of vines coming at them and fires, making it flail as a bolt embedds on it. Zephyra takes advantage of the situation and bites deeply into it.

Initiative order:
21 Thariv
18 Ufeza
17 Zephyra (in melee with the undead)
17 Monstrous undead (118)
14 Blossom (6, in melee with the undead)
11 Luther
  5 Tangle of vines (14)

Blossom goes next.

Luther will fire his longbow at the vines, hoping to find a vital spot and put them down.

Attack: 19 Damage: 8 piercing