Mark looks for any sign of the baddies went.

Elero DC 15 Arcana Check: 19

Mark DC 15 Perception Check: 23

Elero - While the heads are old, they have been carefully preserved, liked prized possessions. One is of the insane subterranean dwarves, the Derro. While the alien being you recognise as a Githyanki. Neither race exists on this plane of existence.

Mark - You see signs of movement in and out of all the doors, but the door to the north has had the most recent movement.

Mark will say, "We should go to the door on the north side, it seems that is where everyone is. Are you both ready to get rid of these thugs." 

Elero takes a minute to pull his gaze away from the creepy statues, and nods. "Let's do this."

You brace yourselves and open the door to the north. You find behind the door some moss filled chambers. These squalid rooms appear to be living quarters, but they contain little in the way of creature comforts and no personal possessions at all. At the far end of the room stands a giant covered in moss, two enormous tusks protrude from its bottom lip, a Troll.

The troll seems to be in some kind of trance, but when you enter the room, its eyes open and dart toward you. It awkwardly gets up and gets ready to pounce at you. Initiative:

Group 1: Erix

Group 2: Troll

Group 3: Mark and Elero

Erix is up. The troll is about to pounce on one of you.

Erix Tries to Thorn whip the troll to get it closer and has his animals attack

Thron whip: 21 for 2 dmg and pulled 10ft forwards

Clumulk:
Ferocity: 1
Bite attack: nat 1
Boar: 
Charge: to hit 6
Land beast: 
Pack tactics (15,18)->18 for 10 dmg.

Erix and his animals charge at the troll and cause some dagame, tearing at the moss around the giant. But on the troll's turn, the moss grows back instantly, and it regains 15hp. The Troll then rends with it's tusks at the Land Beast and at the Boar:

Rend 1 against Land Beast Attack: 21 Damage: 13 Slashing

Rend 2 against Land Beast Attack: 24 Damage: 10 Slashing

Rend 3 against Boar Attack: 9 Damage: 10 Slashing

You all are up. The Troll has regained all the hit points lost in Erix' attack and is thrashing at his pets.

Damage types are important for this creature. So make sure you list what  type of Damage your attacks are.

"The Beast looks like it can heal from normal attacks, how can we beat a thing like that." 

Mark will dip back and hide so he can have the advantage of surprise. 

Stealth: 26

Attack: 10 or Nat 20 for 28 

Damage w/sneak attack: (14 + 7 = 21) or w/ Crit (10 + 16 = 26)

Elero watches the beast regenerate, then kneels in front of the Protector Cannon. "I hope this works," he mutters, as he separates the halves of the shield into wings, removes the energy core from the cannon and replaces it with a small device, which glows red.  Reorienting its housing, Elero converts the Protector Cannon into the Eldritch Flamethrower. "Go get him, little dragon," he says, stepping out of the way as the Cannon marches forward, coming around the side of the troll so its flame doesn't hit friendlies.

Action: Expend a spell slot to create the Eldritch Cannon: Flamethrower.

Bonus Action: walk up to 15 feet to get a clear shot, and blast out a 15-foot cone of flame.  The troll must make a DC 13 Dex save or take 8 fire damage.  Half damage on a successful save.

It doesn't say if we get to keep our last bit of temp HP from the Protector shield, so it's your call DM.

Technically I dismissed the Protector Cannon and created a new one, but my description seemed more thematic.  :)

And the cannon walked up and fired.  Elero stayed back.

Mark's attack Crits. Does that mean that the Troll is asleep? 

Because if so we can wind back the clock a bit depending on what Elero wanted to do.

((What do you mean asleep? ^^))

If Mark's attack somehow put the troll to sleep, Elero definitely wouldn't want to hit it with flame and awaken it again.

(I totally forgot about that feature but I remember that my crossbow can do that. I think you are right DM, the troll should be asleep.)

(Mark received a magic Crossbow with the ability to put a creature asleep on a crit in your first adventure.)

The troll get's hit and falls asleep. You see the moss filled chambers around contain very little in terms of creature comforts. You can search the area, although I'd ask for some Stealth checks so as not to wake the Troll.

((Ah right^^))

Erix orders his animals to stand down, but on guard around the troll while they search the area.

Mark will want to go up close to the troll and search around it. 

Stealth: Nat 20 + 7 = 27

Investigation: 16