Actually, a weapon that could destroy otherwise indestructable demons or megabeasts would be a useful fix to the "only killable by making obsidian or cave-in" monsters that were created by .31.x, like the steam or vomit demons which cannot be bled out or destroyed through any means that doesn't just delete objects.
Still, that's asking for a whole new feature to be implimented just to fix a bug...
If elemental enemies like that naturally exist anyway, though, dwarven lore might well have developed appropriately anti-elemental weaponry purely from nature itself. (Arguments exist that precious few colonies have survived Digging Too Deep, of course.)
So, intrinsic bugs aside, a sword crafted from the purest of glacial ice could be the solution to the fire (and/or steam) monsters, while (specific?) herb-based materials, processed as per rope reed into a cloth blade would do for the vomit variety. It would be based upon an extension of the Spheres idea
And here's the balancing bit: They'd be totally useless against flesh-and-blood beasts, and against the wrong kind of elemental. Cloth swords do nothing but tickle even a marmot, and would be overly susceptible to heat damage.
The best you could really hope for is a long enough lead time to either gather (or import, for such cases as ice-blades for any non-arctic fortress) the specificly required weapons for the rampaging beast, or generate what you can, when you can, for use or trade. You could even set up a special "Monster Squad", prepared with a range of elemental blades and armour that is ready to spring into action when the need arises.
"Oh noes, a vomit monster!" => (s)quads, (p)er person?, choose "The Crushing Of Elements" squad and then "Urist McHerbarmour", to (k)ill from the (l)ist the appropriate entity... McHerbamour has of course got something like plaited Valley Herb leaf clothing, impregnated with Sunberry juice, or somesuch, with a weapon to match... He has a mate, McHotstuff whose magma-plate armour and eternally alight bituminous sword would be an asset in another situation, but could be little or no help in this one. (And sending a fire-sword against a fire-elemental would be like giving the enemy viagra mixed with steroids mixed with every upper and popper available...)
Hmm... Sparring between flammable and flaming weaponry/equipables, and any number of other mismatches that would exist... Should that be hand-waved, or be an additional burden upon the player to micromanage away the probable disasters?
Anyway, that's just an idea. And I still think this sort of thing should be done 'dwarfily' (with due note taken that dwarven nature changes between each and every envisionary of dwarves), rather than ritualistically. Craft, yes; The Craft, no. But that's just a personal preference.