Originally Posted by
Tergon
True, but if you're able to do that, then you've already fought Mirmulnir and, it's highly likely, Sahloknir. And that's assuming you've managed to get this far in the Thieves' Guild questline as well as the main questline, traveling about overland to a large extent, without bumping into any other dragons on the map. The point being, you've proven yourself capable of fighting a dragon already, something that doesn't really involve the stealth skills at all.
For this to not be true, then you hid in the tower while the Whiterun Guards killed Mirmulnir, retrieved the Horn of Jurgen Windcaller by sneaking past all the draugr, were savvy enough to not go and fight Sahloknir, dodged all the dragons that randomly spawn on the map, levelled up Unrelenting Force, and then understood what you were doing in the penultimate Thieves' Guild quest enough to get into the right position to win the battle with your Shout rather than by fighting.