So I got a zombie siege a couple years into my first 40.05 fort, and of course the two necromancer leaders walked right off the map and the zombies have been milling around the edge for a year or so. Hope that gets fixed up sometime. But until then, business has been normal until recently, when the fringes of the zombie cloud have spread out enough to be mildly annoying. So I decided to try to send my smallish military to try to do something about that.
In the next dozen savescums, I learned that even attacking the fringes head-on is deadly, since after clearing out a zombie or two off by themselves, all it takes is one dwarf seeing the next nearest zombie a couple dozen tiles away to spell doom, as he races towards his foe and the dozen or more in sight range behind them. It really is an ugly sight, seeing what was picking off three stragglers outnumbered five-to-one rapidly turn into a frontal assault on the whole almost-as-many-as-my-entire-fort horde.
But then, I found out that military dwarves with no orders and no active alert will flee like civilians.
So what I do, is I move my guys out to pick off stragglers... overwhelm a clump of three to five with superior numbers... then, when the next nearest zombies start to get too numerous for comfort... all squads are now Inactive. Everyone runs away, no casualties. Rinse and Repeat.
I mean, so far I've only offed ten of the horde of sixty, and the main clump is wayyyyy too tight to hope of exterminating them completely anytime soon with my 10 meleedwarves. But it's progress, and now I can regain ground they've subsumed with their random walks.