Grimnir WIS save:19
Grimnir feels suddenly sleepy but manage to shake it off

He then calls out Oorr: "Oorr, I'll take care of Aera. Tell her to bring me up there!"
Grimnir casts  Healing Word on the bird (4 HP), approaches and transforms himself into a little spider (wild shape use:1).

The little spider climbs on Aera, taps it gently with one of its leg and points out the top of the pit. "Time to fly..." though Griminir

Brox tries to hang on when the beholder suddenly moves 1

Then she tries to avoid the dangerous looking ray 15

Forgot Bless for ray: 3

Miraculously, all of you manage to avoid the eye beams, but Brox falls to the ground, taking a bit of damage. 

CON save to keep concentrating on BLESS: 16

Brox misty-steps back on top of the beholder. "You won't get rid of me so easily"

Then she swings Dawnbringer down onto his head repeatedly:

to hit 31 damage 10

to hit 19 damage 14 [ooopps, it considers the hit as 'critical' - please take only the first dice from the damage. So total damage from this attack is 15 not 30]

bEHOLDE

Quote from filcat >>

While Oorr focuses on healing Aera, the rocktopus and the giant spider keep attacking the Beholder.

Oorr ---> Cure Wounds (2nd level) to Aera: 13

Rocktopus: Attack: 18 Damage: 11

Spider: Attack: 20 Damage: 4 + poison damage 11 (DC 11 Constitution saving throw to halve)

Con Save: 17

Belwar follows up the flaming sphere with a fireball. Dex Save DC 16

Dex Save: 6

Fire Damage: 32

The fireball erupts in the upper parts of the chamber. Brox and the Giant Spider are shielded as Belwar sculpts the spell around them. The blast only hits Xazax, burning him black. He looks about ready to drop.

Reinforcements try to get through, but the slick floor prevents them. 

An eyestalk swivels to retaliate on Belwar, casting a green ray at him.
Slowing Ray on Belwar. The targeted creature must succeed on a DC 16 Dexterity saving throw. On a failed save, the target’s speed is halved for 1 minute. In addition, the creature can’t take reactions, and it can take either an action or a bonus action on its turn, not both. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Round 5
21 Xazax (Very Bloodied:176 dmg taken)
20 Belwar/Shield Guardian
19 Grimnir
18 Brox
15 Oorr/Aera
15 Peebles (Dead)
09 Giant Rocktopus (Charmed)
07 Giant Spider 
01 Reinforcements

Xazax still held by the webs begins to thrash about, "Noooooooo... I am... all powerfulllllll..." he screams. Three more eye stalks go off.

Paralyzing Ray on Belwar. The targeted creature must succeed on a DC 16 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Slowing Ray on Oorr. The targeted creature must succeed on a DC 16 Dexterity saving throw. On a failed save, the target’s speed is halved for 1 minute. In addition, the creature can’t take reactions, and it can take either an action or a bonus action on its turn, not both. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Telekinetic Ray on Brox. If the target is a creature, it must succeed on a DC 16 Strength saving throw or the beholder moves it up to 30 feet in any direction. It is restrained by the ray’s telekinetic grip until the start of the beholder’s next turn or until the beholder is incapacitated.

In addition, the walls all around the room sprout arm like appendages that grasp and claw at those near them (next round).

Round 5
21 Xazax (Very Bloodied:176 dmg taken)
20 Belwar/Shield Guardian
19 Grimnir
18 Brox
15 Oorr/Aera
15 Peebles (Dead)
09 Giant Rocktopus (Charmed)
07 Giant Spider 
01 Reinforcements

Brox STR saving throw: 14 + bless: 1

Still in his spider form, riding Aera, Grimnir will ready his action and wait to be close enough to the beholder and jump on it, piercing its skin with his small but deadly fangs.

Bite. 9 + 1 (Bless) > 1 piercing damage, and the target must succeed on a DC 9 Constitution saving throw or take 1 poison damage.

Aera did not move yet (Action lost)

Both Belwar and Oorr feel the world speed up around them as they are slowed. 

Brox is pulled by a telekinetic force off of the Beholder and then pushed 30 feet straight down towards the pit where she stops, held in mid air 10 feet below the lip of the pit.

"Stooooooppp! Or I will droppppp this creatuuuuurre," shouts Xazax.

Round 5
21 Xazax (Very Bloodied:176 dmg taken)
20 Belwar/Shield Guardian
19 Grimnir
18 Brox
15 Oorr/Aera
15 Peebles (Dead)
09 Giant Rocktopus (Charmed)
07 Giant Spider 
01 Reinforcements

With Aera moving up at full speed. Grimnir will ready his action and wait to be close enough to the beholder and jump on it, piercing its skin with his small but deadly fangs.

Bite. 23 + 2 (Bless) > 1 piercing damage, and the target must succeed on a DC 9 Constitution saving throw or take 4 poison damage.

 Brox doesn't hear any of it's pleas. Hanging under the aberration, she flexes her muscles, but cannot move. Her mind tries to work a way around, and without her even realising her mouth forms the words and she misty steps again onto the top of the beholder. 

There, she swings Dawnbringer down on the beast again with all her might:

to hit 22 damage 12

to hit 19 damage 16

edit: add bless to second attack: 2 --> total 19 to hit for second attack

"You have no place on this plane of existence! Only the light rules here! I'll bring it back!"

Grimnir is unable to get his tiny spider teeth through the Beholder's hide. He's scurrying away when Brox steps out of a cloud of mist on top of the Beholder's eye. With a shout and a thrust, she sends the glowing blade of Dawnbringer down through the central eye of Xazax. The beholder screams as his vision goes dark (and a flaming sphere suddenly appears nearby indicating the cone of anti-magic is no longer operational) and then all of they eye stalks go silent and limp.

Oorr comes flying out of the pit, just in time to avoid the now plummeting body of Xazax. He strikes the body with his dagger, cutting off one of the eye stalks. 

Grimnir and Brox, you are falling with the beholder as it heads into the pit! What do you do?!

Brox feels the creature go lifeless below her feet. Oh oh.

11

The limp body of the beholder gives when Brox tries to jump, she loses her balance and only makes it to the edge of the rim, where she hangs on with her hands. 
She tries to pull herself up. 
11

It works and she lies next to the rim, exhausted. 

Feeling the Beholder falling, Grimnir remembers that he saw something in the pit. He reverts back into his Dwarf form and shouts: « I’ll see you in a minute ! »

Grimnir holds on to a few eyestalks and gets ready for the fall.

(reaction removes 30 damage from the fall)

Dex save: 11