There is an Elder Dragon and Frost Dragon atop Skuldafn. There is even an Ancient Dragon in Blackreach.
That would explain why I don't recall ever encountering one. I've beaten the main storyline maybe twice.
And melee has difficulty reaching over terrain, and besides which, dodging and avoiding its attacks while contending with the terrain between you also makes things complicated. It also makes it that much more difficult to stop a dragon's breath attacks.
The side of a hill MIGHT work, but again, keeping the dragon there presents another difficulty. A small patch of trees, maybe, if you can lure it close enough. But that is still risky and they can still get a finishing move off. They can also usually step over most terrain.
The dragon's fire/frost is actually the least worrisome. With some enchanting you can easily negate all damage taken. Throw in a couple resist potions if you're coming short, and you're set.
Unless there is a way to increase your armor rating high enough to counteract finishing moves, then it is nigh impossible. I am currently working my way through the mainquestline to try it, however. I will probably meet the Ancient Dragon before the Elder, however. I am not very optimistic about my chances. I will likely have to break out the daedric for it.
I wouldn't say near impossible. It would require far more patience than anything... I mean, really? ~7 base damage to a creature that has a crazy amount of health and resistance. I'm not well versed in the damage vs. resistance formulas, but it will be a really long fight. And that's an understatement!
Now keep in mind: Sure, you can bypass the death blows/finishing moves (whatever), but the damage you take will necessitate a massive amount of health potions.... I'd guess around 300 or so... and additional mixtures like fortify HA and such...
I wouldn't be upset if I became bored. In many ways, just because something is possible, doesn't mean it has to be done...