Are Dragons under-CR'd?

[frankthedm]

We would all have died, except an amazingly successful dispel magic stripped away all those spells.

That is what you need to do against any spellcasting dragon.

All casters love mage armor and shield. Dispelling a dragon's buffs rarely requires luck since their caster levels are notably lower than the party's Arcane or Divine caster. Unless that caster has chosen to make the mistake of multiclassing. Then there may be no hope to take those buffs down. Practiced Spellcaster won’t be that great of a help since if the character could take it so could the dragon and thus no ground is gained.