First of all thanks for the praise.
A good solution would be to make a hellfire-landmine in the machine farctory and drop it in there. The warlocks will fight as well, but I cant say who would win.
Your idea about the sacrifices is correct, a dwarf with praying labor enabled will go to the temple, run the reaction, and then die. Sadly, I did not convert the reactions (which spawns a boiling rock) to use a dfhack script called autosyndrome, which gives it a 100% success rate. Currently the dwarf has to inhale the gas from the boiling rock, which can happen every time 10 times in a row, or not at all. I wanted to update this for two versions now, and always forget about it. -.-
You can also weaponize your necromancer and route the next siege to him.