Husks cannot be killed except by beheading and bisecting. (And cave-ins and atomsmashing, of course.) I had a guy skill up from no axe skill to high master axedwarf on a single bunny foul fog husk. At the end, it was limbless and gutless and I think noseless and earless, but since apparently it's really hard to behead small things he didn't kill it. (Although I have seen someone behead a cat with a pick.... I'll keep that in mind...)
I just had a look through the interactions etc in my world.sav file-- the interesting thing that I found was that regular reanimated undead and husks/thralls/zombies (yeah, husks and thralls are the same thing, and sometimes they're called "zombies") basically have the exact same tags. The difference, I believe, is that husks don't have to die in order to change their tags. So regular corpses get special corpse-damage code that makes them drop after a bit, regardless of any mortal wounds, and husks don't.
Because the husk interactions do things like add NOBREATHE, and remove blood, that doesn't leave any way for husks to get killed, except bisection and beheading. A husk fluffball would be invulnerable.
Remember that husks weren't affected by the reanimation code, being corrupted living. AFAIK Toady has special hardcoding for reanimated zombie deaths and behaviour.
Actually, I believe the reason that husks are unaffected by reanimation code is that they have the NOT_LIVING token. I wasn't sure about whether they were really undead until I saw that token in all of the interactions creating them. Wierdly, the reanimation effects don't work on things that are NOT_LIVING. As to how they then affect, say, a severed head, I can only guess that a severed head never gains the NOT_LIVING token until it reanimates, and at the point that the reanimated head is struck down, it loses that token.
EDIT: Come to think of it, I'm not sure that it is impossible to reanimate a husk-- I've, for obvious reasons, tried to avoid situations in which husks get created, much less reanimated. Resurrection code isn't supposed to work on werewolves either (that's a forbidden class for the interactions), but it looks like the werewolf corpses lose the syndrome while dead, because they're just body parts at that point, not critters.
EDIT2: Husks leave corpses that reanimate. They're not any more difficult to kill than regular old reanimated corpses though.