Navrine centers herself focusing on the attack, realizing Stonefeathers attack has the beast in a vulnerable state she lashes out with her attacks.

Movement: Standing her ground. 
Action: unarmed strike(advantage), (1,17, +5) 22 to hit. if it hits 5 points of bludgeoning damage.
Bonus Action: unarmed strike(advantage), (7,13, +5) 18 to hit. if it hits 6 points of bludgeoning damage.

Doc closes the distane (he already moved previous round) and takes the fire breath potion:

DC13 dex save or take: 18 fire dmg. (half on succesfull save)

The heroes pressed their attack! 

Chromir swung his mighty greataxe in a wide, devastating arc, but the unwieldy weapon struck only air as the mound twisted unnaturally to avoid the blow.

Navrine darted in, her fists striking true as she pummeled the creature’s vine-laden form, each impact sending tremors through the dense vegetation.

Cassian followed, delivering a precise strike with his longsword. It slashed into the shambling mound, severing thick roots and releasing a foul, earthy stench.

Doc pulled a small vial from his pack and downed the potion within.  He inhaled deeply and unleashed a cone of fire that roared upward toward the towering mound. The flames enveloped the creature’s sodden body, but its waterlogged form resisted the inferno, and the fiery breath caused only minor damage.

The shambling mound bellowed in frustration, thrashing wildly as it fought to free itself from the crushing jaws of Stone Feather's crocodilian form. With a mighty heave, the mound tore itself free, scattering bits of swamp water and tangled vegetation.

But Stone Feather was relentless. The massive crocodile lunged once more, snapping its jaws shut around the creature’s writhing mass. The shambling mound shuddered as the shaman’s bite locked it in place once again, ensuring that it remained vulnerable to the heroes' relentless assault.

Init - PC [HP]; Conditions & Effects; Actions
22 - Chromir [17/29hp]; Greataxe vs SM, a miss!
18 - Navrine [12/17hp]; Two Unarmed Strikes vs SM, both hit! (5hp & 6hp bludgeoning damage)
16 - Cassian [15/23hp]; AC20; Longsword vs SM, a hit! (9hp slashing damage + sap)
03 - Doc [16/19hp]; Move, Drink Potion of Fire Breath, BA: use Fire Breath
-> Because the SM is a tall, large creature, I will say that it can be targeted by the 30 cone of fire without anyone else being in the targeted area (the cone was angled up, in other words, and went over their heads)
-> SM's Dex save (w/ disadv), a success! (9hp fire damage, reduced to 4hp fire damage due to resistance)

Init — Name, AC, HP [hp inflicted]; Conditions & Effects; Actions
10 — Shambling Mound, AC15, 136hp [101hp]; Bloodied, Str check to end grapple, a success!
-> SM is no longer grappled or restrained
03 — Stone Feather, Bite vs SM, a hit! (7hp piercing damage)
-> SM is Grappled and Restrained again
03 — Crocodiles, AC12, 13hp
C2 — Bite vs SM, a hit! (5hp piercing damage)
C3 — Bite vs SM, a miss!

Ongoing Effects
Cassian — Concentrating, Shield of Faith, 10 minutes
Cassian — Antitoxin (adv on saves to avoid/end the poisoned condition), 1 hour remaining
Stone Feather — Concentrating, Conjure Animals, 1 hour remaining
Chromir — Rage, 10 minutes remaining
Doc — Fire Breath, 2 uses remaining, 1 hour remaining

—Cassian has not moved and is still astride Stone Feather.  [The map feature of DDB does weird things when layering tokens and sometimes obscures them.] 

-The hillocks are 10' tall and will require 10' of climbing/jumping to get down.
-All vegetation and water on the lower level counts as difficult terrain.
-Enough large creatures have trampled through the area in the middle (cattails), it is still difficult terrain, but that area is no longer heavily obscured.

Cassian swings again at the SM but misses despite the creature's restrained state.

Chromir uses his momentum from his previous miss to swing round with more power, getting a solid hit in. 

23 to hit, 14 damage. 

Navrine Yells “Keep pouring it on! We can do this!” she lashes out with her attacks.

Movement: Standing her ground.
Action: unarmed strike(advantage), (16,18, +5) 23 to hit. if it hits 7 points of bludgeoning damage.
Bonus Action: unarmed strike(advantage), (15, 3, +5) 20 to hit. if it hits 4 points of bludgeoning damage.

Even though the fire didn't do as much as he hoped, he thinks better use it all and he exhales another fire breath

DC13 dex save for 15 fire damage.

The battle ended with a final, concerted effort from the heroes and the unlikeliest of allies.

Weapons, fists, flame, and crocodilian jaws were brought to bear against the towering foe.  The creature reeled, its movements faltering.

With a wet, gurgling sound, the Shambling Mound collapsed under the combined assault. Its tangled mass of vines, roots, and muck fell apart, slumping into the swamp like a pile of inanimate debris.

Stone Feather, still in his crocodilian form, turned his massive head toward the group. The paladin, perched awkwardly on the shaman's scaled back, shifted to maintain balance as the giant reptile regarded them with unreadable, predatory eyes. The summoned crocodiles, scattered across the battlefield, remained still, their jaws gaping in readiness but showing no signs of hostility.

Slowly, Stone Feather's form began to shift (causing Cassian to dismount). The massive crocodile shrank, its limbs stretching and twisting as scales gave way to leathery hide and sinew. The transformation completed, and the lizardfolk shaman stood before the heroes and retrieved his staff and his headdress of stone feathers.

After placing the headdress upon his brow, the creature spoke and gestured with its staff.   The words were calm and measured ... but were in draconic and completely unintelligible to the group.

Init - PC [HP]; Conditions & Effects; Actions
22 - Chromir [12/29hp]; Greataxe vs SM, a hit! (14hp slashing damage)
18 - Navrine [12/17hp]; Two Unarmed Strikes vs SM, both hit! (11hp bludgeoning damage)
16 - Cassian [15/23hp]; AC20; Longsword vs SM, a miss!
03 - Doc [16/19hp]; no action, BA: Fire breath

Init — Name, AC, HP [hp inflicted]; Conditions & Effects; Actions
10 — Shambling Mound, AC15, 136hp [133hp]; Bloodied, Grappled, Restrained, Two slam attacks )w/ disadv) vs Chromir, one hits! (11hp bludgeoning damage, reduced to 5hp due to resistance)
-> On Doc's init, Dex save (w/ disadv) vs Fire Breath, a failure! (15hp fire damage, reduced to 7hp due to resistance)
03 — Stone Feather, Bite vs SM, a hit! (6hp piercing damage)
-> This attack brings the SM to 0hp remaining ... it is no more.
03 — Crocodiles, AC12, 13hp
C2 — Bite vs SM, a hit! (3hp piercing damage)
C3 — Bite vs SM, a hit! (5hp piercing damage)

Ongoing Effects
Cassian — Concentrating, Shield of Faith, 10 minutes
Cassian — Antitoxin (adv on saves to avoid/end the poisoned condition), 1 hour remaining
Stone Feather — Concentrating, Conjure Animals, 1 hour remaining
Chromir — Rage, 10 minutes remaining
Doc — Fire Breath, 1 uses remaining, 1 hour remaining

Combat ends

Doc will say, as well as doing it in sign language,

"We don't understand Draconic. Sorry. Do you speak common."

Cassian steps off Stone Feather’s back as the shaman returns to his original form, his gaze lingering on the lifeless remains of the Shambling Mound. Relief floods through him as he takes in the battlefield—despite the chaos, they had prevailed with minimal loss. His heart swells with purpose; this is what his oath is for, to protect and unite in the face of danger. As Doc speaks and signs, Cassian bows his head to the shaman in thankfulness for his help. Despite his weariness, his resolve burns brighter, his oath feeling clearer than ever. He will repeat Malrik's message to the lizardfolk unless they can establish another way to communicate.

Navrine looks around at the battlefield and does a quick scan to see if there are any hidden dangers nearby. She whispers “Well done team!” as she too bows respectfully to stonefeather. “I think we could all use a bit of rest and maybe a cup of tea?”

Chromir will use Stone's Endurance as a reaction when the crocodile bites at him, reducing the damage by 10+3 (to 0). Chromir tries speaking the same phrase in both Giant and Dwarvish to see if Stone Feather understands either language.

Cassian will try speaking the message in Halfling as well if Stone Feather doesn't understand Chromir's attempts.

Seeing this Navrine translates the same message in Elvish, and Gnomish as softly and non-threateningly as she can.

Stone Feather remained nearby, his reptilian gaze sharp and appraising. The heroes attempted to communicate, first in the common tongue, then in sign language, and finally in a smattering of other tongues. Each attempt was met with silence or an uncomprehending stare from the lizardfolk shaman.

The draconic phrase and Malrik's name, when tried, were met with an arched browridge and Stone Feather eventually raised a clawed hand that appeared to call for silence.  Then, the lizardman began muttering an incantation. The air shimmered faintly around him as he cast a spell, his draconic voice low and raspy. When he spoke again, his words came clearly to the heroes in the common tongue.

"You should not be here.  The Great Moor is no place for soft-skins."  He said, his tone was matter-of-fact, and more disappointed than angry.

"The one you met earlier ... he carried my warning to you — to stay clear  — though I see now that you did not understand."  The shaman peered past the heroes at the fallen lizardman that had initially come toward the party and the shaman spent a moment regarding his fallen comrade.

Exhaling slowly through his nostrils, the lizardman continued.  "It is no matter, now. What is done is done."

"I am Stone Feather, shaman to my tribe."  The lizardfolk said, changing topics by way of introduction. 

The shaman then gestured toward the pile of debris left behind by the fallen shambling mound.

"Come,"  He said, his voice taking on a somber edge. "Let us talk while we tend to the weed-that-walks."

The group followed his gaze and saw a large, pulsating seed pod nestled within and among the mound’s decaying piles.  Its surface was slick and fibrous.  Stone Feather found a spot in the center of the marsh and began to claw at the earth, digging deep into the muck and pulling up large clumps of wet earth.

The shaman’s disappointment stings, but the paladin understands it; they had stumbled into a world they didn’t fully understand. As Stone Feather introduces himself and gestures toward the remnants of the Shambling Mound, Cassian’s gaze shifts to the grotesque, pulsating seed pod nestled within the muck.

“S-stone Feather,” Cassian says softly, stepping forward. “I’m sorry for the loss of your kin. And f-for not understanding your warning earlier. We only wanted to protect those who might’ve wandered into danger.”

His eyes linger on the seed pod, then on the hole Stone Feather begins to dig. “If there’s anything w-we can do to help you…ensure this thing doesn’t bring more destruction, tell us.”

Nature to recognize seed pod/its properties: 17

Cassian recalled that a shambling mound is driven by its central root-stem, a seed like pod buried within its rotting mass. It is this root-stem that animates the mound and fuels its instinct to smother and consume life. If defeated but left alive, the root-stem can feign death and regrow its body, making the creature a recurring threat if not fully destroyed.

"T-that pod...The Shambling Mound will grow a w-whole new body if we don't destroy it," Cassian murmurs in shock as he recalls the information, watching Stone Feather dig, "You...you knew that creature was here, right? W-were you t-trying to use it in your favor? But it attacked you too. It isn't w-worth the risk."

"It was the Bogmen that raised the mound."  Stone Feather said — displeasure creeping into his voice at the mention of bullywugs — hinting at the struggles between competing factions that shared the Great Moor. 

"The Frog Prince wanted to make war with the lizards."  A glance then at the fallen prince.  "Now he cannot."

"The weed-that-walks has fallen today.  In time, it will grow again, as it is meant to. This is the cycle."

The hole, freshly dug, awaited the seed pod.  Stone Feather gestured toward the heroes, indicating that they were to place the seed pod into the earth.