Assume a dragon, roughly the size of a shire horse. This dragon has a unique breath weapon, that technically "burns" (oxidizes) the target.
Still, I have a hard time imagining how dangerous and damaging would that weapon be against knights (in plate armor) and mundane structures.
A dragon can expel a maximum of 3 liters of aqua regia, after that, they need some time to refill the storage organs. The nitric and hydrochloric acids are kept separate, and only combine shortly before exiting the dragon, and don't even properly combine when the dragon uses their spray mode. Proper aqua regia is only created when the dragon uses the spit or drool modes. Dragons prefer to use their spray mode for combat.
There are two main points to the knight:
- How fast will they die from a direct and a grazing hit?
- Before that, how fast will they be incapacitated from a direct and a grazing hit?