I suspect building-destroyers can't destroy bridges and hatches from below. Make an area with semi-molten rock on the bottom. Fill the area with either water or magma. Occasionally drop the other from above. This should create a falling layer of obsidian that will kill all in its path.
Unfortunately, to work right, there must be either an air gap between the bridge and the fluid below it, where demons will survive, or you have to make the area slightly wider just under the bridge, leaving an area for them to hide.
Another method: This also requires that it's possible to dig a channel through an open hatch. I suspect it isn't.
top:
###
#X##
#^¢#
####
bottom:
###
#>#
####
~XX~
####
The X on the top is an up-down staircase. The ones on the bottom are doors. The one on the left is attached to a lever, and is normally open. The ~ on the left is pressurized water. The ~ on the right is magma, connected to a source, and open to demons. The ^ is a pressure plate connected to the hatch.
When a demon comes along, it will knock down the door on the right, causing water to flood in and solidify the magma it's standing in, encasing it in obsidian.
To reset the trap pull the lever connected to the door, deconstruct the wall south of the up staircase, rebuild the other door, rebuild the wall, draft the dwarf, tell it to stand on the pressure plate, undraft it, and tell it to channel under the hatch. It should leave immediately after, closing the hatch before any demons can get through.
Another possibility: This requires luck, and that building-destroyers only destroy the nearest building.
XXXXXXXXX
X.....#^X
X.....#XX
X... ..X
X......X
X......X
XXXXXXXX
XXXXXXXX
X X
X X
X I ¢ X
X X
X X
XXXXXXXX
The ^ is a pressure plate connected to the hatch. It has water on it. When the water comes off, the hatch closes.
A demon comes up from the hatch. It destroys the pillar. Because the "walls" are floodgates, they don't support the ceiling, causing it to fall and destroy any demons in the area, so long as none are lucky enough to be standing above the hatch. This allows the water to leave the pressure plate, closing the hatch. You can then go down and rebuild everything before refilling the pressure plate.
That will knock any demon lucky enough to be standing on the hatch out, so you might be able to use this opportunity to kill him. Maybe not.