What can a dragon use to increase their armour class?

It is hard to survive as dragon – and yet they do. Despite being known for their nasty claws+bite, fear radiance and breath weapon – thus becoming a high priority target for nearly any ranged attack – they survive past eight centuries. How is this done?

Any long-lasting dragon would fathom bounded accuracy and 150’/600′ range of bows. For example, a single-accidental pass of a small yet well-hidden community of wood elves (‘longbow proficiency + high dexterity = +4 to hit’) requires an AC of 25+, lest this wyrm suddenly become a pincushion.

How does a dragon increase their armour class? Possibilities abound: barding (need it be mithril for flight)? A shield (would it stack with their regular AC)?, retro-fitting magic plate armour (can a dragon wear dragon-scale armour? would-could-should they have a chest plate fitted)? What magic items fit ‘gargantuan’? The list goes on / i have no idea where to begin… i just want a better armour class for a Great Wyrm.

I was hoping this would gain some kind of general answer rather than a slew of over-simplistic questions (‘Could a dragon wear boiled leather barding – or must it be supple due to flexibility required for dragon-flight???’ etc.)