Just make it deep enough to kill 'em all and there won't be anyone alive to land on. Plus, they splash nicely. I had a 10 z-level deep trap that seemed to be 100% effective. Every goblin or troll (or dwarf
) that fell in it was smashed into a torso, a head, two arms and two legs. It was interesting watching it in slow motion and seeing heads, arms & legs fly up 3 or so z-levels above the bottom of the pit after impact. 
I remember reading somewhere that if you make the dodge-me high enough, the creatures falling off the top will explode into bones, meat, and so forth. How high does it have to be to do this? I want to make goblin bone bolts to shoot at more goblins! And stuff like that.
Also be aware that a falling goblin (or anything else) that lands on a living creature is only stunned, not damaged at all. So if a bunch of them come running across, the early ones will hit the ground and break bones, but the second wave may land on the survivors of the first wave and end up unharmed, just stunned for a few moments. So be prepared for some fully functional goblins down there.
Put lots of Weapon Traps down the bottom, to deal with those early ones.
Upright spikes work, too. And don't even need mechanisms!
Actually, they do. Upright spears need to be linked to a lever so they can be raised, and even then it only works if the enemy's already on the tile IIRC, so you can't just have them fall on it.
Actually actually,
A. They start raised, so no mechanisms, and
B. If they fall on spikes while raised, they will indeed take damage.