This mod reduces NPCs damage to dragons by 80% and gives them a damage reduction of 10 to all attacks: weak iron and lesser steel weapons won't be able to harm their steel tough scales and lesser spells will be mostly ineffective.

I couldn't quite found what I was looking for elsewhere on the nexus, they either affected player damage by a serious percentage or didn't lower NPC damage by enough.  I wanted dragons to take no damage at all from small weak attacks, rather than just reduced damage.  Now a single dragon swooping down on riverwood is a serious threat as opposed to a guard arrow pincushion.