Muck launches itself at Redband Bridgette just cursed and engulfs him in goo to initiate a grapple (20 on the check)....

Then forms a facsimile of a face t look towards the others and whisper "Boo."

I'd say half cover for sure. And also prevents them from doing melee attacks unless they get to the other side. Which is either an athletics check to climb over (or prone if they fail), or an action and a lot of movement going round to lift the counter up to get behind. 

Basically it's enough to deter them from wanting melee while there are other targets.

Between the two current actions, ruffian 1 is half health and grappled 

Penley was clinging to the wall above the door with his claws, remaining unseen. He stays up there, digging his claws into the wood. Since he can't use his hands, he whispers an evocation  and a glowing symbol in the form of a sunburst forms in front of his snout. He opens his mouth and roars, the sound unlike his normal voice. The roar takes on a low, ethereal quality as energy is channeled from his voice into the sunburst which sends streaking forth a blast of radiant energy that strikes whichever Red-Brand is closest to the door (which will be 'Rearguard Red-Brand')

Eldritch Blast Attack VS. Rear Guard Red-Brand: 11 to hit. Hit: 10 Radiant damage.

Bok'Shir had ripped into the steak Bridgette had made him with gusto, amazed at how good it was but hardly giving his taste buds a moment to savour it. It was long gone by the time the redbrands appeared. In fact, he had made sure he had invigorated himself to maximum effect by the time they arrived as well.

When the newcomers spot him and call him out as an outsider, a grin spreads across his pointy toothed mouth.

"Clever. And dumb", he says as his companions spring into action. He hops down off the chair he is sat on and straps on his shield, before sending his own bolt of force at the loudmouth redbrand.

He then motions for them to come at him as he draws his sword.

"Glasstaff want you three die, methinks", he proclaims matter-of-factly.

Short Rest Hit Die: 6+2=8
Prior Actions: Cast false life as often as needed to get maximum health using his Fiendish Vigor evocation - 8 Temp Hit Points
Bonus Action: Equip shield
Action: Eldritch Blast against loudmouth - To Hit: 16+5=21, Damage: 8 [force]
Free Action: Draw sword

Loudmouth dies, and so a corpse is grappled. Rearguard is badly injured. Rearguard will attack Penley with his sword (I will say above the door is within reach, but awkward so disadvantage). 12 to hit 4 damage (can't remember everyone's AC but I know someone has a low one)

Middle Guy will attack Bok'shir as he just killed the other one. 19 to hit, 6 damage

OOC: I believe the one with a low ac was Muck. Penley has 16 dex and light armor, so he’s good.

Penley deflects the blow with a defensive blast from his sunburst, then sends out a lethal blast streaking towards the Rearguard

Eldritch Blast Attack VS Rearguard: 20 to hit. Hit: 14 Radiant damage.

OOC: sorry for the lack of formatting. Doing this one on mobile.

You are correct. It is a lethal blast 

"That tickled", Bok'Shir complains as the solid hit was absorbed by his magically enhanced health. He cocks his head to the side and tries to look glum.

"So sad. You alone. Friends dead quick quick. You next", he says with a broad tooth grin at the final comment.

Action: Attack with long sword one handed against Middle Guy - To Hit: 9+5=14, Damage: 2+3=5 [slashing]

If there's anyone else left, Muck will manipulate the grappled corpse's body Weekend at Bernies style and charge at them going "Roar!".

There is one left. Bok'shir has hurt them, but they are still alive 

While puppeteering the corpse, Muck will try to intimidate the ruffian into submission by shouting (in his terrible version of the dead redband's voice) "Ooga booga! These powerful warlocks have taken my soooooooul. Best do what they say or they'll too the same to yooooooou-ooga booga!" It will then manifest an eyeball somewhere in Bridgette's direction to give a conspiratorial wink.

ROLL

A dark patch appears on the front of their trousers. Slowly getting bigger. 

Bridgette left before their turn 

(I don't remember leaving...)

Bridgette looks at the Redbrand for a moment... it seems pretty obvious he isn't going to argue with anything they say at this point...

"I suggest you drop your weapon now... it's probably your only chance to save your life," she says, her tone genuinely sympathetic.

(Bridgette is left to go before their turn would be better wording) 

They drop their weapon. Combat (well initiative) is effectively over 

As soon as they drop their weapon, Bok'Shir points his sword threateningly right at them.

"You boss Glasstaff? How know about us?", he asks, remembering what the loudmouth had said when they spotted him.

While Bok'Shir interrogates, Muck, still deep in character continues manipulating the dead bandit, making him hop around in a gruesome dance.

"Boogaboogaboogabooga"

"We have been told by the Spider to keep adventurers away from Phandalin. So that means we have people watching for new arrivals."

Muck unceremoniously drops the body to the floor and whips its pseudo-head toward the thug. "Wait, you can talk to spiders!? Do you happen to know Tktktktktktk? She's a green orb weaver with a little..." It gesticulates, indicating a stripe down the center of it's body... "... brown thing like this."

"She used to build her web near my puddle back before I was alive..."

Then, as the bandit's words sink in, Muck follows up with. "Wait, why keep adventurers away? We're really nice and super helpful and try not to make people dead unless we really have to."

"Glasstaff is the one who talks to them not the rest of us. I think they are like his boss? We don't know why to keep people away, but we enjoy it so we are happy to oblige"

"Happy to oblidge, huh? Happy to scare innocent townsfolk for no personal gain other than enjoyment? Wow, you guys really are stupid. Are you not getting paid?" Penley sneers slyly, crossing his arms over his chest as he walks up behind Muck, his tall, slender frame towering over the ooze as he tries to taunt the Red-Brands. The sooner they get this over with, the better.

Intimidation check: 23