I use them to create infinitely respawning archery targets.
- Build the necro's cage into a room that is about to be sealed off and hook it to a lever so you can release him after the sealing.
- Put the target area jut outside the cell, on a raised platform. Necro's have a distance limit of about 10 tiles (12?). Beyond that, they can't raise the corpse. Paint a refuse pile that only accepts corpses and have it pull from your main refuse pile.
- Putting the target area on a raised platform with a gap between it and the wall behind ensures that bolts that miss will hit the wall, and fall down to the Z-level below. That maximizes the number of bolts that you can recover and reuse.
- Have a fortification wall looking towards your target area (usually a raised platform that is 3x8).
- Have a drawbridge that you can raise to block the necro's line of sight to the target area. That way you can periodically reclaim bolts / restock the refuse pile with fresh corpses.
- Have a "curtain" wall on each side of the extended drawbridge to prevent marksdwarves from getting line of sight on the necro while the bridge is down.
- Marksdwarf shooting gallery should be on a perpendicular wall to the necro's cell. If you can put the marksdwarves 15-20 tiles away, they'll miss more often and you'll end up with more recoverable bolts in the pit below the raised platform.
- For militia squads, where you don't care if they take time off to train, put their barracks in the same corridor where they need to stand to shoot at the target area. That way, when they go to train while idle, they'll decide to instead shoot at the corpses.
- Marksdwarves shooting at undead do not get bad thoughts about fighting the undead.