Looks good!  I’m still working on it.  Casey would get a bunch of minor things that won’t impact battle much but would be for her life afterwards.   
but she’s also going to get some potions and I’m thinking of a staff for the legendary item.  

Ok Aino will buy a legendary item:  Ring of Three Wishes (no attunement required).

She buys and equips/attunes to a Staff of Power (not legendary, just very rare).
(She unattunes the Cloak of Protection because the Staff does all the same things better and more).

She buys another Ring of Spell Storing (not legendary, just rare) for Thumbtack to hold in her other talon, filling this one with 1 casting of Counterspell and 2 castings of Silvery Barbs for the owl to use as reactions.

She is going to be under the effect of three potions during the fight:
Potion of Speed (very rare, not legendary)
Potion of Heroism (rare, not legendary)
Potion of Cloud Giant Strength (very rare, not legendary)

She also buys but does not yet use 5 Potion of Healing (Supreme).  (very rare, not legendary)

Not a potion (more of a medicine), but she and hopefully the other PCs will also be under the effect of antitoxin.

I'll post what spells Aino casts or preparation in another post, but not tonight!  Too much time trying to research the best items already!

Let me know when yall are all ready. 

A little heads up. My wife could go into labor anytime this week most likely mid week so expect me to disappear for a few days but I'll be back to posting at least once a day once we are settled in back home.

Congrats!  

If you're having a kid, definitely take all the time you need!  No rush on our end.

Best wishes and health to you and your family (especially your wife and little one on the way).

Congratulations! Take all the time you need! 

Aino purchases 8 more non-legendary magic items, 4 of them scrolls with the following spells which Aino casts pre-battle described below:
Mind Blank, Heroes' Feast, True Seeing, Fire Shield.
4 more scrolls she saves to use during battle:  Maze, Illusory Dragon, Forcecage and Mass Suggestion

Aino's pre-battle preparations:
(I will say as an aside that wizards can get crazy and essentially cheat using the spell Simulacrum, but I decided not to do it as Aino as I think it goes beyond the limits of normal optimization).

Casts Water Breathing as a ritual on the whole party (including Thumbtack). Lasts 24 hours.
Casts Comprehend Languages as a 10 minute ritual on herself. Lasts 1 hour.
Casts Rary's Telepathic Bond as a 10 minute ritual on the whole party. We can all communicate telepathically with each other. Lasts 1 hour.
Casts Phantom Steed as a 10 minute ritual. Quasi-real horselike creature appears for either Storm or Kileprat to ride if they wish. 100' move speed along the ground. Aino and Casey can fly, so I assume we won't be using it. Maybe Storm can fly now too so then no one needs it.  It stays at the arena's edge.
Casts Mind Blank on herself from a scroll - nothing can read her mind or charm her for 24 hours.
Casts Heroes' Feast on entire party (including Thumbtack) from a scroll - advantage on all WIS Saves and nothing can frighten.
Casts True Seeing on herself from a scroll - she now has Truesight with range 120' including into the Ethereal Plane for an hour.
Casts Fire Shield on herself from a scroll (choosing cold shield, granting fire resistance) - takes 1 Action, lasts 10 minutes, no concentration.

Thumbtack holds tightly to two rings in her talons, each a Ring of Spell Storing in her claws, the containing 2 Silvery Barbs, 1 Shield and 1 Vortex Warp, the second containing 1 Counterspell and 2 Silvery Barbs.

As described above, Aino quaffs antitoxin (medicine) and drinks the following potions just before battle, in order:
Potion of Cloud Giant Strength (1 hour)
Potion of Heroism (1 hour)
Potion of Speed (1 minute)

On that last turn drinking the final potion, Aino starts her bladesong with a bonus action.

Dam good set up

Storm's Pre Combat Shenanigans:

Buys Shield +3, Bead of Force (Rolling for how many 8 ), 5 potion of healing supreme, and the various potions and scrolls listed below

Casts Find Greater Steed via scroll for a Griffon.

Casts Death Ward on himself, Aino, Casey, Thumbtack and Griffon all via scrolls. This lasts for 8 hours

As described above, Storm quaffs antitoxin (medicine) and drinks the following potions just before battle, in order: (Just copied this from Aino lol)
Potion of Cloud Giant Strength (1 hour)
Potion of Heroism (1 hour)
Potion of Speed (1 minute)

Cool cool

Casey decked herself out with minor magic items, mostly for RP reasons and it was fun.  But some more major items is a legendary staff of the magi, a figurine of wondrous power (griffon) and a spell gem (jade) which she attunes to to put aid in, then uses it the next day.  

precombat buffs:

3 potions:  potion of speed, heroism, and invulnerability 

Summon griffon,  gives her griffon A potion of speed, heroism, and cloud giant strength,  gives her speech to give everyone on the team (even griffon and owl) +21 temp HP.  Then uses her spell gem to cast aid +20 HP and hp max on aino, Storm, and herself.    She casts armor of Agathys (5th level) on herself, (choosing those temp HP over the speech ones)   She casts shillelegh on her staff,  and gives bardic inspiration to the team, even her griffon.   She flies into battle on the griffon, singing and making a big show.

Oh yes same potions Storm takes he also gives to Griffon too.

Ok looks like we are ready. I'll start the into. We are going to do block initiative. Please everyone roll Initiative I'll use the highest.

The party wakes up ready for the final show down. After a night of preping and reflecting, Storm you realize that around Garmuk you get this strange evil feeling, an unbridled rage beyond anything you have felt before. Previously you shock it off but after seeing Garmuk's reaction you think something is off with him.

The party makes there way to the Arena, No Sorath as an escort this time. You are greeted and cheered along the way. It seems most of the town is related to members of the Syndicate and they are real happy you did not murder everyone yesterday. All of the out of towners also cheer you on. 

Arriving at the arena its the biggest festival to date. Even larger then yesterday. There is a sign by the main gates that say free entry, you expect the whole town to be in attendance today.

After finishing your final preparations in the ready room you enter the arena. There Garmuk, Sorath and Dan are waiting. Dan goes to speak but Garmuk quickly silences him.

Garmuk: You Make mind? Join or Die which choose?

Initiative: 14

Casey Init 21.   (Forgot to mention she also drank an antitoxin and gives one to the griffon) 

The final showdown starts playing over the air as tension  builds. Garmuk waiting for your response rage building in his eyes

Aino initiative (at advantage due to Shortsword of Warning):  22

Similar to others, I also forgot potions for Thumbtack:
Potion of Heroism
Potion of Invulnerability
Potion of Speed

She looks at Garmuk for a long time.  "Something has happened to you."

Casey says “Here I’ve ben thinkin this were a friendly fight among friends, and yer not makin sense wit that stuff.  But we intend to win the fight, Mayhap bringing ye back from me buddy Death’s place after if we gots the magic fer it.”

A large battle axe and a 2 hander appears in Garmuk's hands his skin turns red and he grows twice in size and attacks.

Order: Garmuk, Sorath, Aino, Casey, Storm, Dan

Wrong choice.

Garmuk attacks Storm. 

Attack 1 29 to hit 24 damage

Attack 2 37 to hit 20 damage

Frenzy attack: 34 to hit 26 damage

Sorath is up

He cast maze on Aino.

You banish a creature that you can see within range into a labyrinthine demiplane. The target remains there for the duration or until it escapes the maze.

The target can use its action to attempt to escape. When it does so, it makes a DC 20 Intelligence check. If it succeeds, it escapes, and the spell ends (a minotaur or goristro demon automatically succeeds).

When the spell ends, the target reappears in the space it left or, if that space is occupied, in the nearest unoccupied space.

Aino is up

Obviously, on Sorath's turn, Aino Counterspells Sorath's Maze spell as a reaction.

Casting Counterspell at 8th level so no further roll required.

Will post Aino's turn later today (and after Sorath's turn is resolved).