If the only way to kill unkillable demons is with a cave-in, then what someone needs to do is build a cave-in machine. It's remarkably simple, since you can create stone using magma and water. You just have to design it so that the stone is created so that it is existing in open space, which is not so simple, but maybe not impossible. Imagine, if you will, a water filled room with a 3x3 bridge at its center:
##############
#####OOO######
#####OOO######
#####OOO######
##############
And above it sits a channel of flaming hot magma:
################
~~~~~~~~~~~####
###########X####
####### #
####### #
####### #
##########OOO###
Theoretically, you should be able to simultaneously open the bridge and the trapdoor to allow a blob of magma to fall into the pool of water, turn into stone, fall through the open space (its centered over the center square of the 3x3 bridge) and land below as a cave-in. Imagine the fun! The target spot directly below would naturally be ringed with cage traps, to take advantage of the wonderful benefits of dazed cave-in results. An entire system could be built, all automated and timed based on how long the block needs to travel to its target--necessary to save water, which you'll lose in copious amounts. The one drawback, in fact, is the massive amount of water necessary for such a system, but with proper pumping, you could make a fully operational Dwarven Death Star that would destroy or trap any living or non-living thing in the game.
Theoretically.
according to the finest applicable dwarven logic, and previous tests, this WILL work.
Someone sometime back tested what happens when obsidian is forged over a bridge. The result: obsidian wall falls through the bridge. Your bridge doesn't need to be linked, the only issue is that it has a chance of being destroyed by both the obsidian caster and whatever you're trying to keep out. ('keep out' as in you're using this devise to seal a hallway, not just as a weapon)
I believe it was it the art of war thread:
http://www.bay12forums.com/smf/index.php?topic=54532.0 but i haven't reread much of it, could have been in a similar thread.
Assuming your bridge isn't destroyed, you have much less water demand than you're describing.
The issue after all this goes right is, in the case of using this offensively and repetitively, what do you do with the stone blocks? if you just so happen to place it over hell, and by sheer luck, it's over the glowing pits,* it should disappear into the depths. If not, it has to be mined every time it's given space is filled, which isn't necessarily good.
using animals to battle the legions would be fun, someone should try to get a nearly-invincible (or one of the truly invincible) creatures , like forgotten beasts, and chain them along the halls.
* (they do still function as chasms, right? need to start dropping shit on them, but my dorfs won't path down the stairways to my 0-demon-species hell, and the pits aren't underneath the access way. So much for experiments in fortress design and refuse removal.)