Got attacked by a giant blob made of steam. The thing about steam is it's insubstantial, so its attacks were totally harmless. But it was also unkillable. Its body turned red, and the combat log referred to chunks being hacked away (which resulted in harmless clouds of steam jetting out). So, I could either wait for the patch fixing all these completely unkillable things, or I could think dwarven
Obviously my first choice goes to magma. But the nearest magma I'm aware of is 100 zlevels down.
So, option B then, squishing. Now, I'd guess that like demons, atom smashers don't work on forgotten beasts. So, what I did was build a chain with a dog on it, then had the military dwarves try to fall back to it. Then, with the wardog attacking the blob too, I took the squad off active duty. Since they were all thirsty by then they beelined for the well, and rather than chasing, the blob stayed fighting the dog. As I said, its attacks were totally harmless so the dog wasn't in danger (yet).
Next step, dig out a 3x3 square of stone directly above the chain, and channel all around it it! Of course, the damn miner channeled out the last tile from the wrong side, and rode that stone slab down like in Dr. Strangelove. But he was only stunned by his zany antics. The dog and the blob fared somewhat worse for wear, being smashed into paste and all. The blob turned into a pool of water, if anybody's interested.
Grumble grumble, I guess it's time to seal off the caverns until I can rig up the 100+ magma safe pumps required for proper dwarven defenses.