There's a unique guy in the fifth act (forget his name) who uses corps explosion rather effectively. He also freezes anybody who comes close to him and generates new nasties who either blow themselves up in your face, or who you kill and their corpses are then blown up by him...

Nasty nasty stuff.

I think it's going to take atleast two Necromancers and a Barbarian to put an end to that bastard. Well, that or Jaco needs to level up a couple more times.