We did not get a chance to run this exercise, but I'm going to at some point now just to satisfy my curiosity. I really don't think my party of five 9th level characters can beat an adult red dragon in a fight where it's run properly. I don't doubt that a party of five 9th level character CAN kill a red dragon though depending on party makeup.

My particular party isn't the best equipped to deal with dangerous ranged enemies. We really haven't faced that many ranged enemies in our travels so far.

The Bard and Sorcerer have many ranged options, and the Swashbuckler isn't terrible with a crossbow, and as a Protector Aasimar, can fly once a day.
The Fighter has a 16 Dex and wouldn't be terrible with a longbow which he could and should get at the next town.
My Barb usually throws rocks when necessary and does know Chaos Bolt and Mind Sliver although he has never used either so far. I tend to only use spells he can cast on accident or at the very least not fully understanding what he just did, and I haven't figured out how to implement those spells yet.