Got it.  Thanks!  I am ok either way if you change your mind and I'll switch the bonus action below to Clenched Fist to attack instead.

Aino Bonus Action:  Bigby's Hand Interposing Hand.  Creates difficult terrain for the demon wall and provides half cover (+2 AC).

Debatable from spell description whether the half cover just applies to Aino or to everyone... actually it says 'you' so probably just Aino.  The hand does not occupy its own space so party members can step up into its space and still attack.

I agree the half cover is for Aino alone especially if your in the Broom 

Order: Aino, Casey, Storm, Kileprat, Wall

Casey is up 

Casey keeps her concentration on the telekinesis, but isn’t confident it will do anything.

she casts dissonant whispers level 4,   will save DC 20 or take 22 psychic damage and move away from Casey.  

And then strikes at it using her spiritual weapon.  

2 bless plus 16 to hit- if that hits- 10 damage. 

(Move action will be to avoid the wall, she’s on a broom). 

So hit ac 18)

18 is a miss. 17 on wis save w8ll take the damage.

Storm is up

Storm will urge the Phantom Steed forward, "May the radiant light purge this demonic being!"

Bonus Action: Vow of Enmity, Adv on attacks for 1 minute

Attack 1: 27 Damage: 15 slashing + 13 radiant

Attack 2: 23 Damage: 13 slashing + 7 radiant

Attack 1 hit. Attack 2 miss.

As you ride up to the wall the Demon turns to face you. Your first attack connects but as you swing your sword again the demon withdraws into the wall momentarily causing your sword to bounce off harmlessly.

Kileprat is up

OOC:  Wow, that's a high AC.  Not that I have cause to complain with Aino's defensive shenanigans.

Storm, did you add your Bless to your attack rolls?

OOC: It's a wall. Easy to hit hard to damage I guess. 

OOC: yes bless was added and yes 24 or higher AC is going to suck

Hey DM-  just curious- so it failed the save for dissonant whispers, so per the spell, it should run away at full movement from Casey.. But if it only can move forward, does that slow it more?  :) 

OOC: Going to say no on this one. I like it though

DM: Nothing from Kileprat I'll puppet her till she gets back.

Kileprat will attack.

23 and a 25. Miss and hit for 20 damage.

Demon wall has some cracks starting to show.

It will attack storm again. Something about the paladin just rubs it the wrong way 

Two claw attacks and a bite

22, 32, 26 to hit.

First misses rest hits. For 11 damage. And 4 damage.

It will then cast Ice storm.

DC 21 Dex save or take 18 cold damage and 12 Bludgeoning damage.

Half on a save. Area is now difficult terrain.

It moves forward 10 feet.

Top of the order Aino is up

OOC: 15 total for dex save to take it all

Ooh-  casey can reaction cast counterspell with a 4th level spot and block the ice storm. :)

As demon wall starts to cast the spell. Casey pulls out our lute and just starts jamming. The tune is so catchy demon wall's can't concentrate and the spell fails....nice move

OOC: Nice, I put the HP back.

"Good move, Casey!  You beat me to it."  Aino tries to grin at her companion but she is too focused on the abomination in front of her.

Aino and her Bigby's Hand engage the demon wall as Thumbtack flies by to help before flying back out of harm's way.  She is still on her Broom of Flying with 50' fly speed, and Thumbtack and the Bigby's Hand both have 60' fly speed.

Attack 1:  with Booming Blade and Rapier, +2 on the demon wall with advantage due to help, best of 3d20s due to Elven Accuracy.
To hit:  27 + 4 =  31
Rapier, +2 damage:  11 magical piercing.
Booming Blade immediate damage:  8 thunder (max).
Booming Blade secondary damage if the demon wall moves on its own (which I think it will!):  13 more thunder.

Attack 2:  with Rapier, +2 on the demon wall, straight roll.
To hit:  12 + 1 = 13 (absolute min. - nat. 1 plus 1 from Bless)
Rapier, +2 damage:  N/A
(Aino curses in Elvish as she drops her Rapier, +2 and has to swoop down to pick it up as a free action).

Total from both attacks if all damage applies:  11 + 8 + 13 = 32 (11 magical piercing and 21 thunder).

Bonus Action:  Aino has her Bigby's Hand act as an Interposing Hand.  Making it difficult terrain for the demon wall and giving her cover (+2 AC).

Movement - on her Broom of Flying, Aino continues backing up only as much as needed based on the demon wall's movement.

Oh also, @KaylonBalderk - I think you need 2 more DC 10 CON Saves to maintain concentration on Bless.

I added Bless for Aino's attacks but it didn't make a difference this time - 1 hit, 1 miss either way (32 total damage - see previous post).