steeldragons
Steeliest of the dragons
Maybe give the red dragon, for starters, resistance to nonmagical weapons (meaning anything nonmagical deals half-damage)? This pares the rogue's damage down to 20 per round.
Immediately my thought, Klaus. Well, almost...
Make the Dragon the "magical beast" it should always be.
Make Magical beast = can't be hit without a magical weapon. Not half-damage. NO damage!
A 14th level rogue with all of their +'s for this n' that n' feats n' levels n' skills/proficiencies n' whatever else gets thrown in to the "hit roll" (not to mention, as someone has, a Dragon would be, quite literally, the size of the side of the proverbial barn) can and should be able to hit the dragon all damn day. No problem. Probably even land the stones exactly where they want, every single time. I'll even say, at 14th level, I can see the rogue probably able to make "nigh impossible appearing" [to the non-rogues] evasions for half-damage...even easily or almost automatically (natch 1 always fails, always will in my games).
They should not be doing a single HP of damage without a magicked sling stone (or a bless spell or SOMEthing to make it magicky).
Just about every "slaying the dragon story" ever involves the need for a special weapon, either magical in itself or blessed by god (at least) or some kind of other supernatural aid.
From Basic on up we've had gargoyles, many undead, golems (?), night hags (I think), and many others who are IMMUNE to non-magical weapons...How is it 40 years later and dragons STILL aren't counted among them?
If you're going to go kill a dragon...at ANY level...you'd better have something magic to use on it.
Simple. Period. End.
Then the dragon can just keep going after the 14th level rogue, in its various ways, for however many rounds it takes to kill him or for the rogue to get wise that he's not doing a damn bit of damage and [wisely] attempts to get away.