from my understanding Libra identified the weak points and strong points of an enemy. SO an enemy can have blunt resistance. Instead of dragon kick giving and full 10% damage it will give 5% instead vice versa if it was weak agaisnt blunt damage which can give 15% extra damage.
We are all about mitigation so Libra can determine the strongest attacks of an enemy and give a special mitigation towards it like 5% less damage from fire based attacks.