Generally, I've had one heck of a time making anything that isn't a weapon trap, kill things. I've had enemies leap off, down a 40 Z level drop onto up right steel spearsx10 - and basically just get scratched... the whole logistics behind fall damage is absurd, haha.
For traps, use Weapon Traps. Axe heads, Saw Blades, or hell, the weapons of dead siegers work brilliantly on almost anything. Do note, if you play with siegers above 0 skill level (set in GUI) - they WILL avoid traps a ALOT as traps have 0 'skill level' and no way to improve it, that I'm aware of.
If you don't mind a manual 'clean up the pitfall' - you can rig spear traps (assuming your pitfalls are small, narrow areas easily rigging 20-40 spear traps on), wire them to a lever, and then when you get a bunch of morons stuck down there, have a dwarf cackle madly and throw the switch repeatedly. Quite... fun to watch them desperately dodge and run in circles trying to escape the hundreds of spear skewering and tearing them to bits...
If you want fancy-pants, you can rig a simple drowning trap too. 2 z level deep 7 water, then have bridges that close over top the water to trap the buggers below, and watch them drown. Very cheap to do this, but effective clean up of pit falls.
