The Rainbow one kicked you? Please, it could had been worse. if it was a Human you'll receive a rocket propelled grenade in the face. Scales can't save you from a HEAT round! 
Actually Dragon scales are generally said to be very tough, how strong exactly? well they have been known to be resistant to most every weapon, so my guess is there scales have an outer coating of solid carbon, don't even get me started on how strong Carbon can be, so an RPG on a dragon that large might irritate it and push it back about an inch or so but not do any actual damage, in other words
YOUR ARGUMENT IS INVALID!
DRAGONS RULE!!!!
YOUR ARGUMENT IS INVALID!
DRAGONS RULE!!!!
Fine, railgun. Nothing can possibly stop a depleted uranium shell travelling at around 70% speed of light.
Because humans totally have handheld railguns...
Dark matter
Your argument is invalid!
Your argument is invalid!
Since when did dragons learn of dark matter?!
Nevermind - antimatter bomb: 100% mass-energy conversion!
Nevermind - antimatter bomb: 100% mass-energy conversion!
Raven: As far as I know, dragon scales are enchanted to be resilient against every weapon. Except this. Pulls out Excalibur And you ain't takin' it.
This is going nowhere is it?
Yes, let's drop it there.