Rizo and Moni, each lob their fire bolts at the immobilised Bulette.

Attack: 24 Damage: 8

Attack: 16 Damage: 4

The badly injured bulette (21 HP) is fixated on Spark, and tries to eat her.  Attack: 25 Damage: 27

Dodging the bulette's attack, Spark launches two more Magic Stones at it, before making two more.

Attack: 18 Damage: 9

Attack: 19 Damage: 9

The chuul attacks twice more with its claws.

Attack: 9 Damage: 12

Attack: 25 Damage: 11

Bulette's Con save, if it's still alive: 22

Spark's Magic Stones both hit the bulette, weakening it.  The chuul's first claw attack misses, but the second crushes its skull.

Each party member gets 600 XP.

"We have three beans left in this bag.  Ready to plant them?"

“Spark, when we dueled, you fooled me with an illusionary rock behind which you were hiding. Can you do the same here? Perhaps, we could surprise whatever grows out of these beans?”

edit: (and by the way, Spark should get an extra 200 XP for managing the threat)

"The biggest illusion I can make is five feet on each side.  Enough that I can hide behind it, or maybe one of you if you crouch, but probably not all of us.  Or I could put it next to the chasm, so that it appears to block the path.  But that won't help if we fight another bulette."

“Then maybe best you hide. Minty already hides well. I could go to the water stream and make the water move up and let it fall in front of me as if it is a natural waterfall, perhaps that hides me too. Not sure what we can do with the chuul, though. But a surprise attack could be really good against these monsters.”

Rizo heals their chuul for 18 and 18 HP with two enhanced (L2) Cure Wounds. 

(The chuul is now at 53 HP.)

Rizo goes to the bottleneck where a small water stream flows out of the wall into the chasm. There he crouches and forces the water to flow up the wall, then over him and finally falls in a curtain-wide, white spraying water fall into the chasm. Moni waits hidden from view in the chasm. 

Minty, being an excellent lurker, looks around for something to hide behind.

Stealth: 14 

Spark makes an illusion of a rock, shaped roughly like a 5-foot cube, just west of Rizo's location.  It occurs to her that she could hide in the rock just as easily as hiding behind it.  Remembering that she could not see the gateway when she first passed the chasm, she sends the chuul to the top of the ridge where the gateway was, so that it would not be visible to anything that came out of the beans planted near the chasm.

She plants another bean and hides in her illusionary rock.  Bean roll: 36

(The map no longer seems to be available, so I'm going off of what I remember of it.)

A geyser erupts.  Rolling for what comes out of it: 1 3

Ordinary water comes out of the geyser for 3 rounds, and then stops.  Spark plants the next bean in the slightly wetter soil: 4

Another bulette is coming out of the ground.  It does not see the party in their hiding places, but its tremorsense alerts it to any movement in the ground.  How still is the party?

(Because we only need to remain still to stay hidden, we will make Stealth checks with advantage, and then enter initiative.)

Bulette's perception: 19

Spark's stealth: 4

Chuul's stealth: 18

Bulette's initiative: 7

Spark's initiative: 8

Chuul's initiative: 4

14 stealth

14 initiative 

Moni is flying but perhaps the air waves could be noted. 
12 2 stealth with guidance

Minty's Stealth: 28

Minty's initiative: 8

Minty could be a wax figure for his stillness. I guess he also goes first, ready to strike like a mantis.

Initiative order:

21: Minty
16: Rizo
13: Spark
8: Chuul
5: Bulette

Minty and Spark are currently hidden from the bulette.

Minty takes careful aim at the new foe and takes his shot.  

Attack: 26 Damage: 18

Elven accuracy die: 9

He then attempts to find new cover. 

Stealth: 23