So I was well on my way to de-demonizing my place and was about to check out what remained at the pits when a magma man decided to ambush my dwarves. It chased them into my secondary demon trap, a flood room, whereupon it turned around and killed the miner that had unfortunately thought that fleeing behind an enemy would keep his hide in one piece.
I mobilized my military to come crashing down, but while they were moving from one end of the fortress to the other, the magma man apparently didn't like the decor of the place. First he started on the door leading to the rest of my fortress and completely annihiliated it. Then his blazing eyes turned towards the unprotected floodgate.
Since my military was still nowhere near ready, I decided to open the gate and let things flood. The thing is of fire, right? It should freeze into immobility and fall apart, like any fire-oriented creature would do when copious amounts of water cool it down.
Unfortunately, the magma man never had that realization. Because I was shocked, hoping that it may take a longer period of time to die, I forgot to shut off the water rushing through the now destroyed floodgate from the other end (so much for being prepared).
The end result is the entire area is now flooded, with a mere 21 of the 62 dwarves alive sploshing about outside. The magma man is still alive, contemplating the steam rising from the water and the lava river.
Now, is it because the magma man doesn't have to breathe means it can't 'drown' or otherwise be destroyed by water? Because it was a wee bit discouraging that something this relatively small (after the demons were hopefully dead) brought doom down on the fortress.