Well, obviously, that bonfire isn't doing any good.  Time to switch tactics.  I cast create bonfire under the new elemental for 7.  "You lot, keep it there!" 

Eldeth shoots two arrows at the second stone elemental: 

first attack to hit 17 for damage 10

second attack to hit 11 for damage 6

Brox channels the support from Silvanus. She concentrates with all her might to call nature to their help. While standing still 10ft away from the stone monster, she makes vines spring up from the ground, reaching for the creatures feet and restraining its movement. 

Then she calls out: "While these vines hold the monster in place, we can attack it from afar!"

[invoke primeval forces to ensnare a foe. As an action, you can cause spectral vines to spring up and reach for a creature within 10 feet of you that you can see. The creature must succeed on a Strength or Dexterity saving throw (its choice) or be restrained (DC 14). While restrained by the vines, the creature repeats the saving throw at the end of each of its turns. On a success, it frees itself and the vines vanish.]

Healing spirit: 2

Quote from Matthias_von_Schwarzwald >>

Well, obviously, that bonfire isn't doing any good.  Time to switch tactics.  I cast create bonfire under the new elemental for 7.  "You lot, keep it there!" 

 Dex Save vs Bonfire: 13

Fire begins to lick at the feet of the creature, dealing mild damage to it.

Quote from Tiomila >>

Eldeth shoots two arrows at the second stone elemental: 

first attack to hit 23 for damage 9

second attack to hit 10 for damage 4

Brox channels the support from Silvanus. She concentrates with all her might to call nature to their help. While standing still 10ft away from the stone monster, she makes vines spring up from the ground, reaching for the creatures feet and restraining its movement. 

Then she calls out: "While these vines hold the monster in place, we can attack it from afar!"

[invoke primeval forces to ensnare a foe. As an action, you can cause spectral vines to spring up and reach for a creature within 10 feet of you that you can see. The creature must succeed on a Strength or Dexterity saving throw (its choice) or be restrained (DC 14). While restrained by the vines, the creature repeats the saving throw at the end of each of its turns. On a success, it frees itself and the vines vanish.]

 Eldeth's arrows aren't very effective against the rocky elemental, but do a bit of damage to it.

As the elemental tries to step away from the bonfire, vines reach up to grip at it from Brox's prayer.

Strength save vs Vines: 22

The vines rip and then vanish as the strength of the elemental proves too much for them. 

Round 3
22 Grimnir/Jimar
17 Earth Elemental (HP: 115/126)
10 Belwar/Derendil
09 Brox/Eldeth
07 Oorr/Zippy

While directing the healing spirit on the heat of Grimnir, Oorr also commands Zippy to help Brox finding a weak spot in the elemental's defence.

Healing Spirit Tracker: 7.

Unhappy to have lost his last bet, Jimjar shoots 2 arrows from the shadows, and hides again behinds some rocks

Shortbow 1: 16 damage 8
Shortbow 2: 22 damage 7
If 1 attack hit > Sneak attack damage 4
Stealth 15

The spider spirit of Oorr heals Grimnir: + 3 hp (now 35/45 hp)

In front of the large creature, Grimnir attack twice with his magic sword, before disengaging 30 ft away not without shouting at the elemental. "You seem strong but you will shatter. I wonder if we will find some nice gems when you'll break in a thousand pieces."
Amanita 1: 19 damage slashing magical 9
Amanita 2: 26 damage slashing magical 7

Knowing that he is still in range from the elemental, Grimnir will focus on his defence for the next round (bonus action: patient defence, 1 ki point > ATTACK vs Grimnir with Disadventage)
(OOC: I'm trying to give an attack of opportunity to Brox, if the elemental decides to follow me)

EDIT : critic ! + 5 
for a total of 27 damage  if I'm correct (including the half damage from Jimjar)

[I am not close enough. I stayed 10ft away and tried to send my vines, but it didn't  work]

Ooc: oh very well , good thing I did my patient defended then 

lets say I placed myself in a way that Brox is on the path.

Me.........(Brox)..........Elemental

The earth elemental steps out of the bonfire, starting after Grimnir, but as it reaches Brox, it sends two rocky fists at her torso.

1 Slam Attack on Brox: 18 Damage: 15
2 Slam Attack on Brox: 16 Damage: 15

Between the actual and the spiritual shield, the elemental is unable to breach Brox's defense. It grumbles angrily, then turns its attention to Grimnir, but does not go out of Brox's range.

Round 4
22 Grimnir/Jimar
17 Earth Elemental (HP: 88/126)
10 Belwar/Derendil
09 Brox/Eldeth
07 Oorr/Zippy

I light another bonfire.  4

Eldeth shoots another two arrows:

6 for damage 9 

14 for damage 9 

But they just bounce off the elemental without doing any damage. 

Brox swings Dawnbringer over her head:

13 for damage 9 [used the wrong damage roll, should have been 1d8 instead of 1d10].

Again the weapon connects beautifully (+1d8 for savage attack) and the rock crumbles. Quickly Brox infuses her attack with divine energy (divine smite 2d8) doing even more damage: additional damage 20 

Then Brox swings her weapon again. But she put all she had in her first swing and the second misses badly. 

9 for damage 13

{rolling again to hit because I forgot Oorr's help action 12 - still a miss}

(Sorry, on mobile. Hope it's easy to follow. Total damage corrected should be *41*)

Grimnir is in awe.. Waiting for the moment when Brx is glancing at him, he shouts: "what a terrific attack! How do you do this Brox?"

Between blows Brox shouts back at Grimnir: "My god gives me strength, he wants me to bring the light, so I do it!" Then she acknowledges her shining weapon. "Dawnbringer and I are a good team! She is a beautiful weapon, easy to handle!" And with a smile on her face Brox returns to chipping away stony shards from the elemental. 

Keeping the healing spirit on Grimnir's head, Oorr stabs the elemental in the head.

Attack: 13 Damage: 7
Attack: 24 Damage: 5

Ooc: on mobile. Sorry if typos.

Jimjar appears between rocks and shoots 2 arrows before hiding again.

Shortbow 1: 15 damage 3
Shortbow 2: 13 damage 3
If 1 attack hit > Sneak attack damage 9
Stealth 18

The spider spirit of Oorr heals Grimnir: + 2 hp (now 40/45 hp)

 Grimnir runs towards the elemental, attack twice with his magic sword and once with his  free hand  before disengaging 15 ft away.

Amanita 1: 24 damage slashing magical 8
Amanita 2: 24 damage slashing magical 10

Free hand:  25 damage bludg 5

Fire, fists and glowing swords pummel the creature. It stays focused on Brox.

Slam Attack 1 on Brox: 18 Damage: 16
Slam Attack 2 on Brox: 11 Damage: 15

One rocky fist manages to break through her defenses.

Round 5
22 Grimnir/Jimar
17 Earth Elemental (HP: 18/126)
10 Belwar/Derendil
09 Brox/Eldeth
07 Oorr/Zippy

CON save for Brox to keep concentration on 'Shield of Faith': 27

Even though Brox gets hit hard on the head, she manages to overcome the pain referring down her spine and to keep concentrating on her defences. 

"Foolish elemental!  You should've run when you had the chance!  Now die!"  Frostbite for 11. 

Con save vs Frostbite: 8

The earth elemental roars as it realizes the bonfire is still burning under its feet, dealing more damage to it. It manages to resist most of the cold attack, likely in part to the now heated rock.

It's looking very hurt.

Round 5
22 Grimnir/Jimar
17 Earth Elemental (HP: 5/126)
10 Belwar/Derendil
09 Brox/Eldeth
07 Oorr/Zippy

Brox sees the elemental shake and sway. 

She swings twice at them with Dawnbringer: 22 for damage 6 For a moment Brox is distracted by the bonfire burning close to her feet. Then she concentrates on the head of the elemental. She imagines a ball, flying through the air and bursting into a thousand pieces on the other side of the cavern. Then she swings for the second time. 

12 for damage 8 + extra damage for crit 7 + extra damage for savage attack 6 {total damage: 20}

But Dawnbringer isn't her usual maul. It cleanly separates the head from the body of the elemental and it simply drops to the floor, the earth and rock turning into a little mound at Brox's feet. Not showing any resemblance to the former monster it once was.