Setup an ambush where, in order to reach a pile of (not yet animated) corpses, he has to walk over some traps.
A single cage trap suffices. Weapon traps leave the possibility of survival, unless you use metal serrated disks or something.
A method that comes to mind... Reluctantly I might add... Is fighting the undead with the undead. Should you ever happen upon a vampire Dwarf, recruit that sucker into the military and roam your map searching for the necro. If the corpses it puts down come back to life, you're getting closer.
The Vamp has been fighting for a month. He was my solution the the Necromancer
siege I had in the spring, the Necro is in front of my fort, and I just can't find him. I think, I am going to try and move some bodies into my new refuse stockpile, and then fill it with cage traps. I will use my vampire, and the Captain of the Guard (aka. Batman) to herd him towards it. However, it is almost midnight so, we'll see what happens tomorrow.
P.S. In the event that the trap doesn't work, is there an easy way to release a controlled magma attack, from a volcano, with little or no surface access?
Edit: Ooh! Brainstorm! Does magma 'evaporate' or otherwise just go away after a time? Also, will the ground catch on fire? because I am currently relying on a hastily erected wooden palisade for defense.