Total Damage = ( Damage - Opponent Type Resistance ) x Number of Swings
Your opponent's resistances are shown in the upper right of the screen.
Physical Resistance
Light Magic Resistance
Dark Magic Resistance
If you were to use "Frenzied Strikes," a Dark Magic attack with 6 damage on 8 swings against Yoshiro, who has 1 Dark Magic Resistance, the total damage would be (6 - 1) x 8 = 40.
Multi-swing attacks are strong against low resistance, but single-swing attacks do better against higher resistances. If the amount of damage your attack would do is less than the resistance, the attack will be absorbed, dealing no damage.
Most attacks have a base 3% chance to score a critical hit, causing 50% extra damage. A few attacks have much higher crit rates, usually around 25%.