So let me do a general overall commentary:
Opening Round
The party is going against the dragon with 0 prep, no spells cast. That is probably highly unlikely, but this was meant to be a "wing it" test, so I don't mind that. That said, all of them going in super clumped like that against a dragon's breath weapon...I think reasonably minded players would absolutely know to split up against a dragon's breath.
Luca: I think that's a fine opening. Probably more optimal would have been going for blinded with -3d6 SA, as the dragon's save DC against it isn't all that amazing and could have burned LA. But at the end of the day you still need to do the damage so I didn't mind it.
Loonas: You literally can't get more unlucky here, probably one of the biggest dynamics that changed the tone of the fight. If this had failed, the fighter brings a lot of pain rain early on and can tank some of the dragon's big hits.
Dragon: Starts off with a good banish and a fire breath against 3 key targets, solid tactics, makes good sense.
Eilidira: I would have at bare minimum given them mage armor to start the fight (it lasts 8 hours). Sure AC 14 doesn't do anything, but combined with shield thats AC 19. Now again, doesn't seem like much vs +17 attacks BUT when you are talking 3,4,5,6 attacks in a row....suddenly the chance that the AC 19 causes a miss actually becomes pretty high.
The sunburst.... the character is an evoker but there are just so many round 1 options for a wizard. This could have been a wish, a foresight, an invulnerability. Heck even in the damage realm going like a crown of stars could generate more damage in the long run while bypassing saves. I feel like this spell choice was a miss.
Balamur: Good use of heal in this case, makes sense.
Round 2
Luca: Just trying to bring the damage in, no real issue here. I think it was already nothing that Luca shouldn't have been able to withdraw with steady aim, though ultimately didn't really matter.
Loonas: I think this was a good use of banish by the dragon without looking "abusive". Ultimately the dragon didn't do any damage to the fighter but dropped them a nice ways aways forcing them out of the fight for a bit. Ultimately loonas is just having to haul butt back.
Dragon: While the dragon recharging its breath and the players all failing their saves was very unlucky, on the flip side that was a VERY low damage roll for that breath, like REALLY LOW. So the PCs did get some luck back to them on that one.
Elidria: Hold Monster was never going to do anything with the dragon's LA. The wizard needed to make a decision here, either its going to try and blow through the LA with big conditions or just focus on damage, but splitting the difference isn't helping.
Balamur: Blade barrier....its ok but with the 60 foot ceiling the dragon can just fly over it, and the barrier doesnt' stop a breath weapon. I think there were a number of better choices here, even just another good mass heal could have been useful.
Round 3
Luca: Made the save against the blade barrier (another reason that wasn't a great spell to use), but otherwise pretty expected that the dragon's barrage would take Luca to 0 when they are already so hurt.
Loonas: Just doing what he can!
Dragon: The drop of luca on the blade barrier was a nice touch, turning the PCs effects against them.
Elidira: This was a decent choice, providing an auto blinded and deafened along with the damage unless the dragon moves back south or into the various area effects. I like it.
Balamur: This one is a toughie. On the one hand you don't want to leave you buddy at 0, but a healing word instead of a big mass heal or something you are leaving a ton of healing on the table on this one. But I respect the move.
Round 4
Luca: hehe not having a good day
Loonas: Yeah this fighter can bring the pain now that its in the fight, its getting it done.
Dragon: Probably did the party some favors here by takign the two area effects again, though obviously the damage on the cleric was solid.
Elidiar: Morden's sword, oh dear lord. Its time to fire this wizard! I would have taken a high level magic missile instead of this!
Belamur: banish is good spell.
Round 5
Luca: Dragon's LA to take out Elidiar's concentration was solid. Honestly not that unlucky for the wizard, they have only a +2 con and no concentration bonuses whatsoever (fire that wizard!!!).
Loonas: Looking at the screenshot, it looked liek the majority of the attacks hit to me. Also was Loonas getting advantage from his constant inspiration and the fact that when he misses with an attack he gets advantage on the next one?
Ok the fact that the fighter failed his banish TWICE in this fight goes from unlucky to a completely fluke.
Dragon: I like the dragon using banish on the fighter to push them far away, but then using it to bring in the cleric and wizard close for the pain, good use there.
Elidiar: With the might of 9th level magic at their command they use....witch bolt? Fire that wizard!
Belamur: Sorry buddy
Round 6
Dragon: Solid plan to cook both the remaining characters. Basically got an entire initiative round all to themselves.
Round 7
Loonas: Even with so many chips down still almost takes out the dragon in that last round. Just didn't quite have the juice. While 2 crits might seem high, with a 18-20 crit range and a ton of ways to get advantage, I do expect crits quite often from this fighter.
Overall: I think this test shows that the dragon is not a casual pushover, you can't just beer and pretzel your way in and expect to curbstomp. The banish really shined, showcasing how this one simple power actually brought a metric ton of pain and control to this fight. The only character I think "botch" this fight was the wizard, like that was some damn bad wizarding. Most of the other characters I thought fought reasonably.