I flooded my fort with a pressurized water well. Just to give a general sense of my fort setup, it is built around a huge 3 by 3 stairway, with an open shaft (for dropping things) in the middle. This shaft spans from ground level to -97. The majority of my fort is built around the cave at -52 with my farms and such there, with a bit more at -97 for metal forging with the magma sea. The fort design is mostly vertical, with it expanding out no more than a screen at its widest (at the housing section). The lake which supplied the well was at level -26; it was supposed to have a diagonal pressure stop to keep it from exploding, but due to a small error corrected by removing the diagonal and then forgetting to replace it afterwards, there was nothing to slow it down. Within 1 step of opening the doors to let water into the well, the pressurized water from the large underground lake filled most of the z level the well was in (due to the well being at around -40).
By the time a dwarf managed to get to the lever (since the water was coming from above the main living areas and flooding the main stairway), z levels -97 through -53 had been fully flooded, with -52 partially flooded and minor flooding on levels all the way up to about -38. And since the shaft was open to the sky during winter, there is now a massive pillar of ice filling the shaft up to -38. Only about 3 died due in large part to a legendary miner who sacrificed himself while saving most of the city. He was trapped on a level with a cave at -76, and by mining out a wall got rid of all water above that level. Unfortunately for him, the flow of the water (which by the way seems to be much more powerful than before) pushed him down a level and about 20 tiles across an obsidian covered magma pipe (covered from previous attempts at draining by him standing in doorway while mining, releasing small amounts of water) and finally into the pipe itself. He fell to about -100, and survived with only bruising to his upper and lower torse, before finally bleeding to death from the magma he was also drowning in. Through his actions, over 1/3 of the fort's dwarves, trapped in the dining hall, were able to escape.
I have now saved all survivors, and attempting to drain the rest of the fort by opening a hole to the magma sea. And if I'm lucky kill some magma men while I'm at it!
@Talanic: make sure you have an arsenal dwarf set up. Also, I don't think dwarves actually spar anymore. They just do various drills.