i agree with what docb22 said but still they're pretty damn tough ehh? i mean i've seen my hole group of henchmen get ownt by like 4 oni and 2 dredges ¬.¬
It's not that they are hard per say, it's just that they have a tendancy to appear at the worst times... like when I pull a group of upper level Wardens or Dredge. In those situations they are pretty mean and tear apart a hench group. But on their own, I have no problems with the three and four oni spawns.