Iss Sword Muse question.

I have a lvl 85 Iss Sword Muse,equipped with heavy armor but i use knight harmony myself and the enemyes (npcs) take me a lot of hp,i use Shield Song too,and in theory i gain more p.def but not works good and that is what i dont understand. The Iss Sword Muse is not the best Iss at defense????