Ziggy said:
Something tells me you're not used to high level play. A 20th level rogue might have trouble on his own, as the blindsight will make sneak attacks harder, but with the right items (Ring of Blinking etc) that can be overcome. The 20th level fighter will abolutely cream him, he'll have better attack, do more damage and probably have more more hit points as well. Any 20th level fighter worth his salt will also have flying capability and Freedom of Movement, negating the best abilities of the dragon. And the 20th cleric and wizard will just simply destroy him with their DC28+ destruction or Wail of the Banshee.
A party of 13th level character will of course have more problems, that's why his CR is 13

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.Ziggy
You are basing your examples on the fact that the PC is prepared beforehand. If you are going to use the example of a buffed out single class fighter or rogue, then you should at least give the Dragon the same benefit.
And Anyways, flying PCs still can't catch a flying Dragon due to the absurdly high flying speeds of the Dragons. The best shot for the fighter is to catch the Dragon in tight quarters (hopefully squeezed) and have a Ring of freedom of movement (not a given in all campaigns).
It's been proven in these boards (at least to me) that it is possible to beat incredibly tough encounters (EL 6+) with enough preparation and the correct and precise allocation of resources.
The point tha was been made (which I agree on) is that a CR 13 dragon should beat a single class 20th Level, average PC, when the encounter is taken on equal footing. And to qualify this statement even further:
by average PC I mean a core rulebooks PC built using average wealth but also built through campaingning through the 20 Levels. Supposedly (and this is an opinion) the gear you have in Level 20 should include some subpar items you've actually obtained through your own blood and sweat. This is stuff you've collected through the years, not something you got with your Faerun Express at the Adventuerer'R'US back at Waterdeep the week before. The half price rule for selling is there for a reason.
by equal footing I mean that it isn't a "fair" fight if you allow a 20th Level the chance to cherry pick all his options (wealth, feats, etc) and not allow the same for the Dragon. Reverse the roles once in a while and have a CR 13 Dragon do the Scry-buff-teleport on an unsuspecting level 20 PC. See how even they are. Also, unless forewarned, not every caster will have the complete "Dragon-slaying" suite (sp?) of spells prepared beforehand.
And to adress your statement about a spellcaster: Yes, a 20th level caster has a 50% (aproximately based on what others on this thread said) of taking out the dragon with one spell. But therefoer he also has a 50% chance of not killing him. Those aren't particularly good odds in my book.
Basically, fighting a Dragon is unfair. He's a tough monster with some decent spellcasting thrown in. He also has some interesting special qualities. Take into account his excellent perceptions and defenses, and you have one tough mo... That's why fighting Dragons is and always has been about stacking the odds as far possible in your favor. Fair fights are for suckers, and never more so against a Dragon, any Dragon.