I think the current stage of the debuff "Lower Defense" at level 20 only lower defense by -39 is almost unnoticeable when used so I suggestion changing it instead to a % such as lowering the targets defense by 5%. This way it can work on anything and give the same effect depending on the mob/characters strength and become a lot more useful on those higher bosses in dungeons.
Edit: Maybe also consider adding this type of effect to the skills which decrease defense such as Infernal Impact and Quake Cataclysm
Edit: Maybe also consider adding this type of effect to the skills which decrease defense such as Infernal Impact and Quake Cataclysm