Athos will continue to move south and try to position himself next to underneath the flying dragon as to be in striking distance should it fall down to the ground.  He'll then unleash a guiding bolt at the flying dragon.

Dirty 20 to hit, for 14 radiant damage and the next attack has advantage.

After taking the full brunt of the blast in the face, Gimmond was start to realize that maybe he should've taken Zumfir's offer of protection. "Tch! Too late now!" He roared as he flew to the east, hoping to if nothing else make it harder for the dragon to get the right angle as he laid into the beast once more. But even after catching his second wind and burying the pain, his still flinched from the worse possible moment.

Dex Save[DC 14]: 7. Failure.

Ba: Second Wind, Regaining 10 Hp. 
Ac: Attack Dragon with a most vicious spear!
Attack #1:  One-Handed Vicious Spear vs Dragon(AC ??) - To Hit: 20 For: 19 Piercing Damage. 
Attack #2:  One-Handed Vicious Spear vs Dragon(AC ??) - To Hit: 9(Nat 1) For: - Piercing Damage. 

Mo: Reposition to east of dragon.

[I'm a bit confused, how is the dragon able to catch Twinkle in a 40 foot line when it is 60 feet above her?]

Becuase I failed to take into account height and just measured on the map to see if I could catch two people... So just on Gimmond I guess (and would have been better off maybe not using it and using standard attacks) 

Zumfir stares at the dragon. His allies didn't need any immediate healing . . . so, maybe he could do something to help the meele fighters get in range. He whispered a few words, but for the dragon, it was a loud command, filling his mind.

FALL.

Casting Command to try and force the dragon to stop flying and stay on ground. (It's kind of a custom command since it's not on the list, so if i can't do that i will use the Grovel command.)

DC 16 or follow command

Twinkle keeps the half dragon occupied with a barrage of punches, hopping back 30 feet (so 45 feet total) towards the northeast at the end of her barrage, so as not to present an easy target.  

Action:
Attack 1: 29 (Crit) damage: 14 force
Attack 2: 28, damage: 9 force

Bonus Action [Flurry of Blows]:  
Attack 3: 23, damage: 8 force
Attack 4: 25, damage: 10 force

Looking at the new command, it doesn't accept custom commands. So will be grovel. (and I will resolve the round shortly when I get slightly more time). Just posting to say this is not forgotten 

Athos and Gimmond both hit the dragon once each. Command (which would have been good) does not take hold, with a 21 save.

Twinkle then massively injured the half dragon. Moving to try not be a threat but having left it on 1hp, has made herself the biggest target.

The half dragon will naturally target Twinkle. It takes a massive leap, moving 30ft closer, before running the remaining 15ft. It takes two attacks, both just hitting (both 17) for a total of 14 slashing and 14 fire damage (not evenly split between the attacks, but ended up at the same number coincidentally)

The dragon will take swipes at Gimmond. The first is a crit for 18 slashing damage. The second is a nat 1. And the final one misses (16)

Everyone is up 

Even with only of the dragon's attacks hitting its mark, Gimmond is nevertheless pushed well past the point of being bloodied. Even taking a moment to center himself could do little negate his grim reality. He tries to press forward and keep up the attack -- to keep the dragon's attention. Yet his every prod and lunge is so exhausting an affair that nothing landed cleanly without sliding off the dragon's hide. 

Ba: Second Wind, Regaining 8 Hp. 
Ac: Attack Dragon with a most vicious spear!
Attack #1:  One-Handed Vicious Spear vs Dragon(AC ??) - To Hit: 14 For: - Piercing Damage. 
Attack #2:  One-Handed Vicious Spear vs Dragon(AC ??) - To Hit: 14 For: - Piercing Damage. 

Athos seeing the half dragon chase after Twinkle, will release a second level guiding bolt with a 21 to hit for 18 radiant damage at the half dragon and charge after it with his full movement.

*Next attack against the half dragon gets advantage.

As per my summary, it was left on 1hp. So the next attack does not need advantage. The half dragon is dead 

Seeing that Gimmond and Twinkle were severly injured, Zumfir loudly prays for the safety of his comrades- Powerful healing energy courses through him, Travelling through his words and entering his friend's hearts.

Cast: Mass Healing Word Healing: 18 (raw healing) + 6 (Disciple of life) = 24 total healing for everyone

(Question: Can i use Niko's mace to summon a celestial? or does it fall into the 'Only one spell per round' Rules? )

To sscourge: So long as you're not spending a spell slot to summon the celestial, you're fine. The rule now is:

One Spell with a Spell Slot per Turn

On a turn, you can expend only one spell slot to cast a spell. This rule means you can’t, for example, cast a spell with a spell slot using the Magic action and another one using a Bonus Action on the same turn.

(Thank you! In that case, i will use one charge of Niko's mace to summon a celestial.)

With a prayer, a celestial, taking the form of a dancing moon maiden with a longbow appears, ready to take on the dragon.

Casting: Summon Celestial

Selected Avenger celestial: he will have the Radiant bow