You should get combat reports about him fighting. Use these to identify his species. Then, mod members of that species (assuming he's not a dwarf) to instantly evaporate at any temperature, reload your game, and use the one-step function until clouds of boiling whatever appear. Then, locate all of the clouds that were where no visible creature or item was, and make a note of it. Those places are where you should focus your search after you savescum and remove the instant-death mod.
Alternatively, keep soldiers stationed in a perimeter ~25 tiles, with no more than ~10 or so tiles between each soldier, around any reanimatable parts until they're all gone. The necromancer will flee from the soldiers, thus preventing them from raising anything while you clear them, then you'll be able to let him sneak into the confined spaces of your fort where you can actually corner, identify, and kill him.
It's practically impossible to locate a necromancer on an open field without dozens of dwarves to form a search party. Since necromancers seem to only be detectable in a ~3 tile radius and actively flee from any dwarf they come within ~15 tiles of, you won't find them unless you corner them, and you won't corner them in an open area unless you have the numbers to make a tight net that they cannot slip through. Even that's not reliable.