How many level 9 PCs needed to be able to kill an adult red dragon?

[Unoriginal]

I'm open to suggestions on what I could have and/or should have done differently.

The Adult Green Dragon could have used the Wing Attack Legendary Action to attempt to get out of the grapple, no?

Not to mention it'd let the Dragon get further away from the PCs when they pile on their big melee attacks regardless of if the PCs beat the save or not.

I The casters are likely toast if they don't Absorb Elements

I mean, Absorb Element does nothing against a Green Dragon's poison.

Beyond what Aimeryan said (which is essentially hit and run tactics to the max), 1 or 2 spells on the Dragon makes a huge difference. Plain an old invisibility is the perfect spell for a predator like a dragon. It has an hour long duration, which means its prey isn't going to see it coming until it's far too late. At a minimum this would mean no pre-buffing.

Also, the starting combat range feels off. The only way things start that close is if the Dragon is ambushing the group (or somehow got ambushed). If the Dragon isn't using something like invisibility it should be visible from a long way off. Which plays in to the hit and run tactics mentioned above.

Even without adding any spell to the Dragon or making them focus on hit-and-run, I have to note that the Dragon in the simulations (especially the first one) mostly unleashed melee attacks on the two targets who were the most apt at enduring them.

CTurbo, I'd say it may be worth considering the same fight, but the Dragon picks the Rogue or the Sorcerer as priority for the melee attacks. Or maybe even the Bard.