I gave you the numbers - there's nothing complicated about them. Level 20 mercy monk with bracers of defence, eldritch claw tattoo, and ring of protection. Bog standard feat choices - ability upgrades to max (so dex/wisdom 24/24). I don't think you need to see the character sheet - all I did was take my level 12 character and bump them to level 20. Standard array to start.
There's nothing it can do to the monk at 100 feet, either. Like, at all. And since, as I started with, these are mental exercises that I do when I have insomnia, "what would happen if a dragon flew 100' above my monk at level 20?" isn't going to help me much. I do exercises like, "could my level 12 monk solo a storm giant?" (yes, easily) or, when this stat block came out, "hmmm...what about that dragon?"
If the dragon was immune to poison, it would be a different story, though the monk might still win. I should math it out with an elemental monk, since that takes flying off the table for the dragon. Long story short: high level monks are OP! IMO.