You could try the Harm spell...this puts his hitpoints donw to 1 if the spell is successful. After that you shouldnt have much trouble to defeat him.
I think you need to lower his resistance a bit and cast a greater malison to stand a chance...even then I dont know if it will work but I do now it works well on Greater Wolfweres and trolls that too got a bitchy regeneration
And hey it even works on dragons some time which IMO is far tougher then Irenicus...