Damage calculation in battle.

Well, you're exaggerating a bit. For this to happen, Tazar needs to be level 50:
0.3 (defense cap) * 0.7 (Expert Shield) * (1-0.15-50*0.05*0.15) (Defense + Specialization) = 0.1
At a typical level of 20, this results in approximately 15% damage, but only if your unit's defense is 28 higher than the enemy's attack – this is only relevant against neutral creatures.

If we analyze this point in detail, we should consider one nuance. The fact is that when the maximum difference between your hero's defense and the enemy's attack is reached (as seen in the formula you provided – the coefficient of 0.3), no spell that modifies defense – including Expert Shield – will have any effect. The total damage will still not fall below 0.3. Only the secondary skill Defense (and Tazar's specialization, of course) can reduce this value:

0.3 (defense cap) * (1-0.15-50*0.05*0.15) (Defense + Specialization) = 0.1425

To achieve a damage reduction to 10%, the hero needs to reach level 69. And, accordingly, in the case of maximum hero development (let's assume level 108 is the maximum), the damage will be only 1.20%... Now that would be a tank! :) Although, this point should still be checked somehow...