Ok, so Cerdis still moves to get Rizo and Justin in one line (to the square shouth of Justin). Both Rizo and Lily get an attack of opportunity first.

(Lily is too far away for her aao but Rizo was waiting for that.) 

Attack: 14 Damage: 4

Rizo Dex save: 20

That hits! You hear a little "yelp" from Cerdis.

Rizo ducks out of the Acid Breath and only takes 11 dmg.

Justin DEX save: 3 

Oof!! Justin is looking REAL hurt!

Rizo just managed to get behind Justin to avoid the brunt of the acidic spray. Nevertheless, having not quite recovered from their previous encounters, he does not look much better than Justin.

In an attempt to boost Justin's spirit, after the acid settles, he calls out: "We need to keep him busy so that the others can do their job! We can do it!"

Minty realizes he is useless in the healing department so he concentrates on contributing to the demise of Cerdis instead. He takes careful aim and fires his crossbow.

Attack: 18 Damage: 8

Sneak Damage: 9

Lan sees Minty's bolt penetrate Cerdis' scales easily. Determined, he follows Minty's example, carefully aims at Cerdis and fires his crossbow, too. For all he knows he did exactly the same as Minty.

Attack: 20 Damage: 4

Minty turns to Lan. "Nicely done."

The broad smile of an accomplished apprentice welcomes Minty's comment. 

The Dragon now looks really worried! He's going to bolt if you let him.

2 pools of water surge outward in a grasping tide. Rizo and Justin,  need you to make Strength Checks.

Justin: 18

Strength Check Rizo: 15

Yeah, we both fine. 

Justin swings his Warhammer wildly, hoping to finish the little dragon off

Attack: 21 Damage: 5  

Rizo closes in to Cerdis next to Justin, engulfs his club in flames (Shillelagh) and strikes at the dragon attempting non-lethal damage. 

Attack: 20 Damage: 6

Lily just waits and avoids any harm (dodge).

(I’ve been asked to take over the end of this encounter.)

When Cerdis glanced about for any possible escape routes, Justin’s Warhammer struck true and took the last air out of Cerdis. With his demise, the magic of the very young black dragon’s lair begins to fade, too, and you notice how small rocks begin to be falling from the ceiling. You feel that this cave will collapse soon and that you have to act quickly if you want to do anything inside of it. You may only have a few minutes left.

Rizo checks Cerdis’s life signs and laments, “He’s dead.” 

Minty and Rizo (and everyone who passes a DC 10 Perception check):

(Everyone inside the cave will roll a D100 at the end of their post, on a roll of a 1 you take 1d6 bludgeon damage from a larger fraction of the cave ceiling falling on you. This value of 1 will increase by 1 every 6 seconds (after each move + action + bonus action that you take), i.e. next round everyone can get 1d6 damage on a roll of 2 or lower, and so on.)

(Are we sticking with the same turn order: Egan, Lilgrea, Minty, Lan, Justin, Rizo?)

(I think it would be enough to just keep it one turn at a time, everyone can just say what they intend to do in their turn and we organise it from there. I think the exact order does not matter much and we can go faster if everyone just posts when they have time instead of a prescribed order. Rizo is checking life signs of the dragon this turn, so he won't do anything else.)

Egan chooses to Dash for the entrance, in the hopes of saving the chest.  He also yells at the severely wounded Justin and Minty, "Get out of there!"  He sincerely doubts either of them will listen.

Does Egan's 30 feet of movement get him within range of the falling rock?  If so, I'll roll the d100.

Justin casts Cure Wounds on himself and uses 30 ft of movement to get to the chest.

I'm not running when you are risking your lives!

Cure wounds: 8

D100: 80

I've checked the map, and I'll say Egan's just inside, so I'm rolling.

83