The chuul winces at the damage inflicted by the treant (46/79). Rizo and Moni keep up the artillery fire and continue to incinerate the living tree.

Attack (Rizo): 14 Damage: 24
Attack (Moni): 23 Damage: 10

In the last moment, the treant dodges out of the way of Rizo's Fire Bolt but it gets caught by Moni's Flame Seed (42/111). 

Initiative (distance to treant): 18 Chuul (5 feet), 16 Treant, 15 Rizo (35 feet), 13 Spark (35 feet), 7 Minty (10 feet)

Spark advances 10 feet and slings a Magic Stone at the treant.  She makes three more and launches another one.

Attack: 15 Damage: 10

Attack: 14 Damage: 5.5

The chuul attacks two more times with its pincers.

Attack: 17 Damage: 5.5

Attack: 10 Damage: 5.5

Minty gets his feet under him and slows down his motions to concentrate on the new feeling of the whip in his hands as he strikes out at the tree.

Attack: 21 Damage: Unable to parse dice roll.

Elven accuracy die: 10

For some reason that rolled at disadvantage rather than advantage.  It to hit should be a 23, but I believe a 16 hits anyway.

Regular damage: 8

Sneak damage: 8

OK

10

9

That is regular and sneak respectively.

Spark’s stones miss the treant by a hair’s width but the chuul takes advantage of the treant’s evasion to grab a branch with its pincers. Hold by the chuul, the treant can’t evade Minty’s whip and the strongly enhanced weapon cuts deep into its bark (19/111). Now desperate, the treant trashes at the chuul in hopes to free itself while Rizo and Moni Launch another volley of fire bolts.

Con save treant against chuul poison from tentacles, DC 13: 24 (if this fails, the treant would be paralyses and Minty’s hit would be a critical and Spark would have advantage, both together would likely kill the treant, which we would assume)

Attack: 23 Damage: 14

Attack: 18 Damage: 15

Attack (Rizo): 24 Damage: 20

Attack (Moni): 14 Damage: 8. 

The treant resists the poison from the chuul’s tentacles and lands heavy hits at the chuul, nearly killing it (17/76). But then it bursts aflame from another fire bolt and the chuul disposes the lifeless burning tree with a wide throw into the chasm where you can see it smoldering for a few more minutes before the little water flow extinguishes the flames.

(1667 XP for everyone. Anyone else wants to roll for a few beans and manage the outcomes?)

Spark takes another one of the beans and plants it on the other side of the chasm, then returns after watering it.

Bean roll: 61

Rizo’s Initiative will be: 12. 

A bulette emerges from the ground and starts charging at the party.

Spark's initiative: 12

Chuul's initiative: 1

Bulette's initiative: 13

(I guess it doesn’t hurt to do Rizo’s turn now when I have time.)

Rizo lobs a Fire Bolt at the bullette and instructs Moni to release hers once it gets into range (assuming we are again at 70 feet distance).

Attack (Rizo): 12 Damage: 4

Attack (Moni’s readied action): 9 Damage: 9

Minty's Initiative: 16

Minty sees the charging Bullette and redraws his bow hoping to slow it down a bit.

Attack: 25 Damage: Unable to parse dice roll.

Elven advantage die: 19

Normal damage: 10

Sneak damage: 11 

(I guess I'm playing the bulette as well?)

The bulette starts with 90 HP.  With all of 2 Int, it dashes towards the group as quickly as possible, jumping over the chasm and stopping in front of the group since it lacks an action to attack with this turn.

Not wanting to provoke an opportunity atttack from the bulette, Spark remains in melee range, hurling two Magic Stones from close up.

Attack: 14 Damage: 8

Attack: 10 Damage: 12

She makes more Magic Stones.

The chuul attacks the bulette with its claws.

Attack: 16 Damage: 8

Attack: 8 Damage: 9

Spark misses the bulette, but the chuul is able to grab the bulette with both of its claws.

Bulette's Con save: 11

The bulette avoids the chuul's poison, but is grappled.

Initiative:

21: Minty
16: Rizo
13: Bulette
12: Spark
1: Chuul

Well, it is slowed now.  Minty takes aim and shoots a bolt at the Bulette.

Attack: 27 Damage: 39

Elven accuracy die: 27