Ok, let's look at the CoC damaging the Dragonborn then. The Dragonborn is dressed in his legendary dragonbone armor, enchanted with resist magic to the 80% cap and also reduce the cost of Restoration spells by 100%. He's also at level 81, and has invested each level up into health.
The CoC hits the Dragonborn with a sword. The dragonborn's armor takes most of it, and his high health is hardly damaged. Even if he does take significant damage, his 0-cost restoration spells quickly recover his health. Or he could use Become Ethere, another powerful shout, to become totally immune to damage.
CoC fires a spell at the Dragonborn. 80% resist magic takes care of that. An arrow? Restoration or become ethereal. Dragonborn could use the sneak perk that causes an enemy to stop combat for a moment when sneak is entered. Dragonborn turns invisible, sneaks away. Uses throw voice, another good shout, to distract CoC. Calls Dragon - while the dragon can't harm CoC, it can distract him. Plus the dragon can die, as he's essential.
A well-aimed sneak attack and the CoC takes significant damage. Could probably throw a paralysis poison on that arrow too.
Perhaps we should say the battle will never be won?
Dragonborn can reach 80% Physical Damage Reduction from 567 Armor Rating, and is capped at 85% Resist Magic. The CoC can reach 80% Physical Damage Reduction from Armor, can reach 100% Reflect Damage making him completely immune to melee attacks, and can also reach 100% Resist Magic/Reflect Magic. The CoC can lower the Dragonborn's resists with Weakness to Magic/Element/Poison spells, making him vulnerable. The CoC can also have 100% Chameleon, making him completely undetectable, while able to score sneak attack after sneak attack after sneak attack, all of which bypass Armor Rating with 100 Sneak perk. The CoC can also Silence the Dragonborn, preventing him from Shouting or casting spells. The CoC can also cast spells while wielding a Two Hander or a Weapon+Shield, the Dragonborn cannot.