I found necro's weaken is better that the Soul Cleaver one on my char "build", so I use necro to solo most of the bosses.
About the PVP, I made the mistake to put my Exalted Soul Cleaver to rank 10 before knowing about the passive lv 10 skill, so I don't like using it, because there is a smal but possible chance(0.1% or 0.01%) of dying on each attack.