I want to do this, but I'm unsure of the specifics. Is drowning needed? Because I think I know how you could do this without tons of water.
1. Long drop onto your fortress' finest spikes, preferably bronze, iron, steel, or *gasp* adamantine. Kill your livestock instantly.
2. Pull a lever to lower the spikes, and let the necromancer see the animals. Let him reanimate them.
3. Pull it again to raise the spikes, re-killing the animals and blocking the necro.
- Is drowning required so the animals are uninjured? If so, disregard everything I said :I
- You could remove the spikes and raise the drop to reduce overall injury, but that makes killing the zombies hard again, right?
- Can necromancers see through solid glass walls? Nobody agrees on this. (Or, a fortification in front of a window would work better)