How can I kill a goblin in a pit?

I have a goblin in a 5 z-level deep pit. The problem is that all of my dwarves get scared when they come close to him and drop whatever they're doing, even though he is behind fortifications. I would like to have him killed, except every time I try to remove the fortifications to have my military kill him, the carpenter gets terrified and runs off.

I don't have lava. How can I kill him?

Drop something more dangerous than the goblin in the pit.

Note this may not actually solve your problem. A better option may be to set your military to practice just on the other side of the wall, so they'll be there, then have your miners dig through the wall (not the fortifications, since the miners will run from that side of the wall when they see the monster). Otherwise, if you have lots of bolts, just set your ranged squad to kill him and wait for them to get a shot or two through the fortifications.