Tharanau looks at Tryn and gives him a nod of approval. All or nothing! He then sends again lightning towards the creature, hoping to whittle down its fighting energy. 
When Tharanau looks at Progo and how to help him, to his surprise his new found friend is not lying on the floor dying, but is up again and joining the fight once more. Tharanau is relieved. 

Damage 2 witch bolt

Progo gets the sneak attack bonus since Mimic is within 5ft. of Tharanau so Engineered_Beard roll 1d6 at any moment to add it up. 

Goraseth quickly puts himself in front of the mimic anticipated route and attacks, but Mimic unexpectedly turns to Tharanau avoiding the attack, which in turn works for his others attackers as Tryncaryn smashes it with quarter staff seemingly breaking on of the boards of its body and tries to follow with feet, but it did not go as good. Progo's Arrow hit and wedged itself between two boards possibly injuring interior of the monster.

Mimic Tries to turn the tables in this ambush and withstanding the blows reaches for another victim not yet realizing that his first victim avoided death by sheer strong will, and not willing to give up. Mimic launches it's pseudopod in hope to end this electrifying torture and attacks Tharanau. Unfortunately for the Mimic, Tharanu chooses that moment to surge his current again and elctrifies him again what makes pseudopod to retract a bit and allowing Tharanau to easy avoid it. (9 to attack - miss). Mimic then continues his roll and rolls over to the corridor entrance(A6). (Movement provokes opportunity attack from Tharanau.

(Shouldn’t we, Seth and Tryn get an AoO when the mimic turns away towards Tharanau? It is leaving the range of both. Tryn moved next to it last turn, I believe.) 

oh right sorry for that, he does not leave seth range but Tryn does get an opportunity attack as well

Sneak damage 5

(Good catch, not sure how I forgot about that!)

Tryn exclaims, “We’ve almost got it! It’s already turning to flee!” And he strikes again with his quarterstaff putting all his weight in the blow (attack 14 for 4 damage).

(OOC: This was the AoO from the Mimic's turn, it is still Goraseth's turn.)

Goraseth sees the group is holding together and hurls his weapon on the mimic. (Attack: 17 , damage 4).
If the attack lands, he will call upon his deity and activate Divine Smite (+ 7 radiant damage) "Bahamut ! Help me slay this monstrosity !"

Tryn follows the mimic (to B7) and  joins in Goraseth’s onslaught. He batters the monstrous barrel of ill intent with his quarterstaff before kicking it for good measure, too (hit 12 for 9 damage and hit 23 for 5 damage, all mundane bludgeoning). 

"Come back and fight Progo!" Progo stumbles up to C5 and makes sure he gets a clear shoot at the barrel. 

Attack: 10 Damage: 5 plus 6 sneak

Mimic barraged with damage turns into a formless sludge to the point where Progos arrow dips in it and stays, then the sludge turns in ugly soup and soaks in the floor.

(Mimic is defeated)

"It appears that barrel has sprung a leak. That's what happens when you try to eat Progo." (Is there anything left as proof we found and killed it, out side of the giant bite marks and acid stains Progo has!)

Goraseth smiles, showing big white fangs. "Well, this was a battle ! I love this ! And Tharanau, what sorcery was that ? I never saw such magic, a leash that electrocutes your enemy ! I am impress I must say... We need to make sure this will be tell at the Inn in Phandalin !". He then comes closer to the half-orc "Progo, it seems you took quite a beating. Do you want me to try to close your wounds. It seems my connection with my god grew this last weeks, and he gave me the power to heal people to a certain limit."

"Well Goraseth, it's innate, it's a bit crazy. Took me a while to control my powers." For a moment Tharanau's face turns sad, then he controls himself again. 
"The damage varies, but it's a good spell to chip away at bigger foes." 
Then he also turns to the others: "Progo, I am glad you're alive. I saw your limp body and thought you had died. And Tryn, crazy fast footwork there! Great stuff!"

"It will take more then a barrel with teeth to get the best of Progo. Besides the worse shape we appear,  the more dramatic, and believable we are of our great clash with our foe. Hard to show stain on the floor as proof of dead hiding creature. No proof, no pay. Also if job seemed to easy, they might not want to pay big reward"

After a few step Progi stops and looks back. "Maybe a little wound reduction wouldn't hurt the image too bad"

"Well done, everyone!" Tryn exclaims. "The king must have been quite lucky to escape that monster! Let's get back to him and report and I agree, a few bite marks might help to convince them that we dealt with it." 

(Using 5 healing point of Lay Hands on Progo)
"I agree, let's bring back this gelatinous corpse to the King and announce to him that he was not insane, there was a monster !". Goraseth starts to walks, dragging the body of the mimic.

You manage to scoop some of that gooey liquid, but there is barely a handful left of it, that you can carry making a bowl out of your hands tucked together. Hardly it does look like a man-eating monster, but it is or at least was. 

As you arrive at the Throne room King asks you from behind the door:

"Is the matter settled, is the monster slain? are we safe now?"

"The deed is done," announces Tryn. "We found a monster disguised as barrel in your storage room. It tried to wolf down one of us whole, but Progo escaped by a hair's width! This must have been the monster that assailed you, too! And we killed it!"

Progo looks a little embarrassed at the mention of his state during the fight. "It wasn't that close for Progo."