Potential line: Ignore 15% Magic Def/Physical Def when attacking

Say a monster has 40% defense. With 15% def ignore, your damage to the monster would be:

damage = (damage before defense)*(1-(0,4*(1-(def ignore))) = (damage before defense)*(1-(0,4*(1-0,15)) =
= (damage before defense)*(1-0,34) = (damage before defense)*0,66

So it would be a 0,66/0,6 damage improvement, or 10%

That's counting that none of your skills ignore part or all monster defense.

PS: There are no more 2 types of defense, the only defense is the defense rate.