Tunnel out to beneath the necro, dig out small room part way along and set as corpse pile. put doors at entrance but don't forbid. Create max length bridge (make sure it raises and lowers along the tunnel) set to constant repeat with a lever in room with most dwarves and connect it to the bridge. enable mining on a useless dwarf and force him to be the one to breach the surface next to the necromancer. If possible, get him back inside safely but that's why it should be a useless one. Now I'm assuming it's not a self raising biome, so with a "clear" line to the corpse pile the necromancer should path through the bridge. I haven't tried this in the latest version, but if a bridge is repeating up and down things SHOULD still try to walk over it and thus keep dying to it. Since the necromancer will be the closet thing to the entrance you create, he should go down first to try and raise the corpse pile (recommended doors as I'm not fully aware of how their raise dead works/range on it) and get smashed by the bridge. If you don't want to risk a dwarf dying, use the bridge+ramp method people use to safely breach lava (someone can probably explain it better than I could). Probably want to wall this trap off from your fort at one point as well to be on the safe side.
Assuming this would work fine, what do you think guys?