Yolov procures a flask of oil and is ready to hurl alongside a fire attack.

Valentine lashes with her whip again. This time it manages to get beneath a couple of plates, causing one to be lifted slightly with the strike. (Chaos Creature: 9 damage.)

Coseris, you're up!

Initiative (Round 3)

Coseris: 

Question for DM: Seeing Bankhan and Mordecai squeezing together up front, are they blocking attacks from behind towards the creature? Since I am a "Small" creature can I get within the same space as them with my movement to have a clear attack?

Movement 25ft - using as much is needed to get a clear shot. or No movement if I can take a shot without disadvantage 

Action: Attack using Fire Bolt seeing Yolov prepare the flask.  Attack: 14 Damage: 10

Movement: Using remaining movement if any to retreat to the back of the group. or just staying put if I didn't use any movement at beginning of the turn.  

Yolov gives a sharp nod as he launches the oil flask at the creature... or tries to. >.>

Oil Flask Attack: 16 Damage: 4.
(if it can be considered proficient, then add +2. Otherwise take the improv. weapon result)

(@All Nailed it! And in addition, the creature becomes covered in oil. If inflicted with any fire damage(such as the above firebolt), the creature will take an additional 5 fire damage as per item's description. It doesn't specify if this damage is persists, so up to dm fait.)

Corseris releases a bolt of flame, as Yolov hurls the flask of oil alongside it. The flame splashes onto the carapace, burning the face of the beast as the oil flask shatters and ignites the front half of the creature. (Chaos Creature: 26 damage.) The creature looks nastily burnt from the flames.

(Note: Throwing an oil flask is an improvised attack. The damage roll is 1 + your ability modifier, not 1d4. Taking Tavern Brawler ups it to 1d4 however. Oil flask damage from fire is one-shot, effectively the oil is all used up to deal 5 fire damage. It is a 1sp item, so give it some credit.)

The Chaos Creature opens its maw again as it spews out a spray of acidic breath at those ahead of it. The ground lightly dissolves beneath the feet of those there. (10 foot cone)
Acid Spray vs Valentine, Bankahn, and Mordercai: DC 12 Dexterity Saving throw (Bankahn and Mordercai have disadvantage due to squeezing), taking 5 acid damage on a failure, or half as much damage on a success. On a failure, you also see your feet get stuck in the sinking ground, causing your speed to drop to 0 until you use an action to dislodge yourself.

The Chaos Creature then attempts to back up slightly, but its back strikes against the Barrier, preventing it from burrowing away. (No, there's no AoOs as it never left the weapon reach)

Vax, Warx, Bankahn, Mordercai, you're up!

Initiative (Round 3)

Having now exhausted his supply of spells, Vax tosses another Firebolt at the creature.

Fire Bolt:Attack: 9 Damage: 1

Save: 5

Though unable to dodge out of the way, Bankahn uses his reaction to cast Absorb Elements, raising his knife and cutting a glyph of warding into the air. Most of the acid evaporates before hitting Bankahn. The rest is absorbed by the spell of Heroism he is under.

"Humph. Ready for you that time," Bankahn grumbles, stabbing at the creature again with a knife now glowing green with acidic energy.

Booming Blade Attack: 12 Damage: 3 and 3 thunder damage on a willing move.

(I forgot, the knife would also do a 4 acid damage, if it hits on...a 13....)

Dex save: 1

Mordecai, being in such close range, reaches out with a gloves hand, palm outward. Closing his eyes for a moment, a green pulse leave his hand (inflict wounds)

attack: 16, damage: 13

if at disadvantage because of the tight space, use the lower of the two attacks: 21

Gritting his teeth at the acid stream, Mordecai then stumbles back, away from the creature, giving Bankahn room to work. "Call out when you need to switch!"

Vax releases a mote of flame again, however it misses its mark of the sweet spot the other shot managed.

Bankahn weathers the acidic spray by using his magical capabilities. The heroism spell manages to prevent any true damage from being done, and refreshes its defences as he acts.He attempts to stab the lobster-like creature again, but because he is fighting for space with Mordecai his strike falls short. (13 misses, so even if you rolled disadvantage it wouldn't hit.)

Mordecai pulls upon negative energy, holding his hand out as he thrusts it at the face of the lobster. The creature attempts to avoid the strike, but it is slow and relies on its tough casing to protect it. The spell strikes into the weak spot opened up by the fire combo as the necrotic blow causes devastating harm to it. (Chaos Creature: 49 damage.) It is on its last legs.

Warx, your go!

Initiative (Round 3)

OOC: Sorry for the delay.

Warx watches as his companions unleash a flurry of attacks against the creature. Seeing it take damage and trying to retreat, he decides to press the attack and add an Acid Splash on top of the already charred front end of the being. "Save some of the fun for me." he calls out.

Spell Attack: Target must succeed on a Dexterity saving throw  (DC 13) or take  6 acid damage. 

Warx decides to fight acid with acid as he releases a volatile spell at the lobster-like creature.

Chaos Creature's Dexterity Saving Throw: 18

While the creature seems resilient to the acid, it hits true. It appeared to be rearing for a final attack, ready to blast out more acid. The speed of the spell is too much for it as it smacks into its mouth and face, causing it to dissolve those features. An unearthly scream comes from it as it flails upon the final straw before collapsing before the group. (Chaos Creature: defeated! 50 damage.)

Congratulations! You have defeated the monster!

Valentine cheers with a small squee then bounce on place a little before she wants to take a look at the creature. 

'' These creatures are soo strange! Wow! But we did it! really good!~ ''

Yolov could not have rushed forward fast enough to fallen creature, another vial in hand and an wide-brimmed smile. "Excellent, excellent, excellent. More research materials! Wonder if this one will dissolve as well? Or will it be reborn anew? Oh! So many questions to answer." His smile somehow manages to widen as he starts scooping up some pieces in separate vials: Skin. Mucus. Carapace. Whatever he break off into sample sizes at the very least and store away.

"Oh, if only we had one of those haversacks or bags of holding." He says wistfully. "Hm. I wonder if this is last one?" 

Vax searches around to see if there is any trace of more entropic beings or whether the party can finally get out of Mechanus.

Investigation:18

Yolov goes over to start taking samples. As he prods at it, the body suddenly swells up and them explodes! Guts, bile, and other black ichor paints the tunnel and its inhabitants (no damage, just gross). From what the party can tell, they have eradicated all known entropic creatures present on Mechanus that Mordenkainen sent them to deal with.

Bankahn raises a hand, muttering magic in an attempt to shield himself from the glop.

"Can someone clean this up? We can now return to Mordenkainen, but I'd rather not do so looking like this."

Coseris, being small used the others to keep the guts and innards from reaching him. steps out from everyone clean as a whistle, "well that doesn't look pleasant."   He holds out a waterskin to those who would need to wash themselves off, "I'm afraid I do not have magical means of cleaning this is all I can offer."

On the way back out of the tunnel and past where the other creature was felled, I would like to look around for any of the Bolts I shot as we walk, Not an intense search just keeping an eye out. (There would have been 3 total) investigation : 14

(Eight wizards, someone must've taken Prestidigitation :-p )

(@Poke Well you're not wrong Poke..)

"Oh my." Yolov says, sliding his goggles down just time to at least protect them. "Hm! Note to self: Entropic beings continue to remain volatile even upon expiration. Inversely, this may increase the possibility of harvesting past normal decay rates. Will have to test the limits of these possibilities." He rambles on absently as he peeled off organs to start testing their elasticity and the like.

"Hm.. hmm... Most interesting. Misfortunate. What some of you may desire I can potentially provide. However, such a process would be rather time consuming as it only covers 1 cubic feet per casting. Perhaps if I stretch the limits of it.. hmm... hmmm..... Another project to consider! Pile the organs here and here. And oh -- here. More vials. See if you can push a few into here and I will take care of the rest." He starts handing out the last few vials he has on his person to each caught in the splash zone. The whole time after he stares rubbing his hands with eager glee. Or perhaps in preparation to cast Prestidigitation? Hard to tell with Yolov, but he gets around to doing it provided the others don't wish to Plane Shift sooner than later.  

Wiping off his face and hands with Coseris's offered water, Mordecai approaches Yolov. "Say, if you have a moment, could you take the time to clean off my armor before we leave? I fear this gunk may be corrosive. I must say, though, that it seems like such a waste to kill these creatures. Out of place they might have been, but surely they didn't deserve to die. In the future, ought we not to preserve life where possible? Perhaps it is not beyond our abilities, combined or otherwise to return such, er, unusual specimens, to whatever entropic corner of reality they dwell in. And where did these things come fr-" Mordecai pauses for a moment, before continuing on a different thought. "Is this all we were sent to do? Was this the great task of Mordekainen? To slay a few sorry creatures far from home?"