Might be a good plan, but Liarin is the same size. How'd she going to get though the tunnels?

The Ettin attacks Liarin with his morningstar and battleaxe:

Morningstar Attack: 10 Damage: 17 Piercing

Attack: 20 Damage: 14

Y’all are up

Avery continues the attack:    Attack:  14      Damage: 10

The sword bites into the Ettin's side, and one of the heads looks towards Avery.

Xanrym, Theldrin, and Liarin are up.

Xanrym continues to mock the ettin: "You'd expect one head would get the brains and the other, the beauty.  In your case, it seems like neither head got either!"  

Xanrym cannot see Avery from where he is, and he shouts, "Avery, can you make it back to the tunnel?"  He glances to the north behind him and Theldrin to see whether they have enough space to back up and make room for a dire wolf.

Movement: holding his ground this round since it sounds like neither Liarin nor Avery have started moving out of the ettin's room
Action: Cast spell Vicious Mockery on the ettin, WIS 13 save or target takes damage (4 psychic) and has disadvantage on the next roll it makes before the end of its next turn.
Free Action: Talking

(OOC: Does Theldrin have a good enough view of the Ettin for a ranged attack with Fire Bolt?)

Yes he does

Theldrin casts Fire Bolt sending the flaming bolt over Liarin and towards the left head of the Ettin.

Attack: 10 Damage: 9

Liarin tries savaging the ettin. Attack: 13 Damage: 11 DC 13 STr or prone

(OOC: Don't forget that Dire Wolf still has advantage from pack tactics until Liarin decides to retreat to the tunnels and fight while squeezing into that space.)

5 incase of a crit

Ettin's Save against VM: 19

Ettin's Save Against Prone: 12

Liarin's Advantage toll to see if she crits: 20

Assuming your all still in the room with the Ettin:

This Ettin is looking more beat now, but still looking ok. One head swings a Morningstar at Liarin with disadvantage, the other attacks Avery with the Battleaxe:

Attack against Liarin: 24 Damage: 10 Piercing

Attack against Avery: 18 Damage: 20 Slashing

You all are up.

Seeing the Ettin's punishing attacks, Xanrym makes another call for the party to fall back and lure the ettin into the narrow hallways where he hopes that it will be less able to wield its weapons. 

• Xanrym shouts into the ettin's room, "Fall back and lure it back into the confined hallways!  It's the only way!"  Avery could probably safely disengage.  Liarin probably has to choose between disengaging but being within the ettin's reach if it follows here or dashing north and risking an opportunity attack.
• Xanrym casts vicious mockery at the ettin to try to debuff any opportunity attack it makes: "Let's find out whether you're a giant coward or a giant idiot!"
• Then Xanrym tries to ensure that they split up effectively and get out of the ettin's immediate reach.  If it follows them into the narrow hallway, they can reengage it from both sides next round.  
  • He calls out to Avery: "Avery, back toward the door.  Protect Theldrin."
  • And to Liarin: "Liarin, straight backward to me, up the northern hallway."
  • Xanrym heads north.  As he passes Theldrin, he grabs the wizards arm briefly, points down the eastern tunnel, the way that they had just come, saying, "Head back the way we came and north, past the door, so that it can't get to you easily."

Once Xanrym is in position, he sets his lantern lantern down on the ground in the corner.  He'll need his hands free to use his crossbow once the ettin is in the hallway, and his primary concern right now is getting everyone else far enough away that the ettin cannot reach them this round without dashing.  Once the monster is in the narrow hallways, they can attack it from both directions at once.

Movement: Moving north to the end of the hallway where it turns (i.e., 4 squares north of Theldrin's current position).
Action: Cast spell Vicious Mockery at the ettin, WIS 13 save or target takes damage (3 psychic) and has disadvantage on the next roll it makes before the end of its next turn.
Free Action: Talking, Item interaction - setting down lamp

(OOC: I suspect that both setting down the lamp and equipping the crossbow will consume his action.  If not, he'll cast vicious mockery on the ettin, "Let's find out if you're scared or stupid."

No you can do all those things no worries

Avery will do a slashing attack against the Ettin's leg as he moves past and covers Theldrin, falling back towards the door. Attack: 10  Damage 13

The Ettin save against VM: 2

It will still take a swipe at Avery as an Opportunity Attack, even if it's distracted by Xanrym's confusing taunts:

Battleaxe Attack: Attack: 17 Damage: 14

Had to edit cause i forgot to add disadvantage

Theldrin and Liarin are up

Liarin makes a fighting retreat, snapping at the ettins legs as she backs up.

Bite: Attack: 7 Damage: 11 Piercing, DC 13 Str Save or knocked prone