It’s an interesting thought experiment and I really hope somebody comes up with a good plan before it’s my turn several years from now.
Cave-ins are classic vanilla DF solutions but the only “undead” ever faced is a vampire and they’re not all bad (so to speak).
1) How do you kill an undead? By definition, you can’t so it becomes
2) How do you
get rid of an undead? (Or thousands thereof.)
2a) Direct confrontation:
2a.1) Golems evidently don’t cut it. We could sacrifice for a Guardian.
2a.2) The cave-in/domino effect. The Arena is not set up for testing this, perhaps somebody could try this with the save? Does anyone have actual experience dropping rocks on an undead? My own experience comes from setting up magma elevators and something unexpected
always happened. I’m not ducking the work but it would be better if somebody else did this.
2a.3) The White Mage. From planning through execution to finish, how long would this take? And since there can only be one White Witch Hunter at a time, is it possible to have a Mage-in-training to take over relatively quickly if the first one isn’t enough? And by relatively quickly, the manual says “after a month.” Would that be one White Mage with multiple Lifemancers? At the Library of Mind and Body, “Study Sorcery” grants lvl5 – so is that kind of an upgrade to a new White Witch Hunter?
2b) “Get rid of ‘em,” à la Timeless Bob’s Water Balloon extravaganza. If floods can knock things around, can they be flushed off-screen? Water staff waves only target three at a time but waterlings give infinite amounts until deconstructed, right? Then a controlled cave-in could seal the breach and restore FPS. But since many of them are holed up within those walled rooms, that could be problematic. Or perhaps a water staff could immobilize the front lines of undead in ice, allowing the Witch Hunter to do his thing at leisure. Brings to mind the scene in Eddings’
Belgariad where Riva Iron-Grip is using the Orb to pop Grolims one at a time on the frozen river.
As was pointed out, Mages are not a quick fix and require many costly reagents. I suggest exploratory miners to find gold asap, inviting caravans and diplomats to order Golden Geese and a Trade License as a contingency, and researching Alchemy.