Minty is tempted to touch the amulet.  The thought of talking to the dead intrigues him, though he can't quite figure out why anyone would want do do so.  However, there are more immediate issues at hand.  He sneaks forward with the others, helping Spark as best he can.

 17

(With Bappy's and Minty's stealthy check, total 24, we already get safely to 60 feet without being noticed - black in map. If we arrive to a total of 40 with Rizo's and Spark's roll we won't be noticed at 30 feet distance - blue on map - and have a surprise round. Otherwise combat will start normally at the 30 feet positions.)

Rizo Stealth: 17 Guidance: 2

Slowly and silently you approach the large boulder behind which you expect to find the small group of goblins. You also keep an eye on the crevice where you had spotted a chuul and keep out of sight of the aberration. At around 70 feet to the wall, you peek around a smaller boulder and finally spot the goblins again. They are at 60 feet distance to you and are busy preparing for a ritual with an elongated, slightly curved rod that looks as if made of bone or a light coloured wood. One of the goblins (#1) is on the look out for the chuul, while the other two organise various spell implements and chant, it looks like one of them (#2) is leading the ritual while the other (#3) assists. At this point, the chuul seems unaware of the goings on. You figure that the goblins are, indeed, about to subdue the chuul as you had thought before. You aren't familiar about the ritual, but it seems that at this stage a line of sight is not required. The goblin's are heavily armoured and it could be that they are prepared to battle the creature as part of the ritual but it could also be that that isn't necessary, you are unsure. If you let them finish, you may have to fight the combined force, but also if a battle commences so close to the chuul, it will certainly be alarmed. Considering your options, you draw closer...

Map (please, let me know if you wish a different formation for the player characters, I put it like this based on your options on the character sheet):

(We reached a total of 40 already without Spark's roll, so the stealthy approach to a 30 feet distance was successful.) 

Quietly you round the corner of the goblins' large boulder and find them still at the same positions. But there has been a development, they stopped the incantation and the rod has begun to glow very dimly in a dark red light. The goblin that lead the incantation (#2) has picked it up carefully and whispers something to their companion (#1) in their language. You still don't notice any indication that the chuul is affected or aware of anything. You are ready to strike and you are very close to the goblins. Should they chance a glance in your direction, they might discover you and spoil your surprise, or they might still get a hold on the chuul and make the fight much harder. You can try to strike them down quick and claim the rod but they look tough themselves and a battle might alarm the chuul, too. You could also try to intimidate, deceive or persuade them, claiming that the chuul is your quarry, but it is hard to tell whether they will fall you in the back midfight, unless you get hold of the chuul-taming-rod in some way. Entering in dialogue will certainly concede your element of surprise but given that you managed to sneak up on them shows that you pose a serious threat that they might consider to avoid. 

(There are a number of possible avenues and I'll hold here for 48h - unless everyone says that they're ready to proceed - so that you can all give some input on your strategy before committing to any action. Feel free to ask questions if I wasn't clear and you are unsure about something. We assume that - in game - your talk about the strategy takes place while you move from 60 to 30 feet distance.)

(edited to correct spelling)

Spark's Stealth (if Minty helps her to remove disadvantage): 11

"Perhaps I could use an illusion to distract them."

"I fear I can't help more than providing some magical insight to something that you want to try. Well, that is outside of combat of course."

"I can try to get the drop on the one with the rod."

"So I'll create the sound of a screaming goblin over there, and then we attack while they're distracted?  And try to get the rod?"

"That might work. You mean to trick them into walking behind the boulder and to get some more distance from the chuul in order to not risk that it will attack us while we fight them?" Rizo asks for clarification. Then he looks expectantly to Bappy, "What do you think? Any other ideas?"

"Sounds like a plan to me!"

"Great! Let's do it," Rizo exclaims enthusiastically and almost too loudly. "How about we lure them behind this big boulder so that the chuul hopefully doesn't hear us. If you create the sound here (G14) they should here it and the boulder still blocks it largely from the chuul. You two could hide a few steps back here (G19 and G20) to take them in surprise. Minty and me can hide behind the smaller boulder (A7 and B8) and simultaneously attack them in the back. Or do you wish to draw them out even further, Spark? With some luck we get the rod and can claim the chuul for us!" 

(We assume a perception check to hear the scream and the fighting will be needed for each hex away from source, with a mod of 5 if there is an obstacle, so that makes it a DC 19 perception check for the chuul, the check would be more difficult if you draw the goblins further out but they will start to make spell saves for the second illusion onward. Shall we proceed with the plan? I'll draft a map with the proposed positions once you let me know where you wish to hide.)

"I think drawing them here should work fine."  Spark points to the nearby ground (G14).

Having all agreed on a plan, you go into hiding and Spark creates the illusionary screams of a goblin in distress. You hope that the real goblins will fall for your ruse and that you can overcome them quickly. You all have time to ready an action that you might be able to take when a goblin comes into your view. 

Rizo touches Bappy to provide a small boon for the beginning of the ambush. Then he takes his place and prepares his wildfire club.

(Last chance to adjust your positions if you don't like the ones I chose. Minty can hide in "plain sight" because he is proficient, even expert, in Stealth. Everyone else would have to make a contested roll if you want to hide in the goblins' view. )

 Map:

Minty is good.

Bappy is ok where he is

After casting Minor Illusion to create the sound of a screaming goblin, Spark makes some Magic Stones and prepares to sling one if a goblin approaches.

Minty (You're the only one with a line of sight):

At the first scream, the goblins freeze. Then, they look quizzingly at each other. The one with the rod gives an instruction to the one that had assisted him before, pointing roughly in your direction, then he turns to the other and points towards the crevice behind the boulder. The assistant moves urgent but careful steps towards the sound, followed by the rod-bearer. 

 

Spark and Bappy: You wait in anticipation for the goblins to fall for you distraction, you don't see them and can only hope that they react the way you planned. You don't hear the tree goblins over the screams, so have little choice than simply stay put until they appear. After a few moments, you begin to ask yourself if it really works, but then it appears a goblin at the position of the illusion. You recognise him as the one that assisted the one with the rod before. He looks around dumbfounded trying to sort out where exactly the screams come from (Investigation: 15 vs DC 15).

Rizo patiently observes Minty for a cue that he should round the small boulder and join to spring their trap.

(Spark, you may release your stone at the goblin in sight (#3) in reaction to his appearance. Additionally, we start a surprise round where everyone may take a turn and roll initiative for the ensuing combat.)

Map:

Attack: 24 Damage: 7

Spark's Initiative: 7

In the surprise round, Spark will launch her other two Magic Stones at #3.  If #3 is killed on or before her turn, she will run to G15 and launch any remaining stones at #2.

Attack: 27 Damage: 6

Attack: 16 Damage: 9

Also, Spark will use her bonus action to make three more Magic Stones.

Just realized I forgot to add rite damage to my attacks, so I'm rolling it now: 2 6 3 (added to the respective damage rolls if they hit)