So I've got this forgotten beast made of lignite swimming around in the cavern lake. There are a number of factors conspiring to make this the single most annoying thing I have ever had to deal with in DF.
One, he is right up next to the main staircase. This would not be a problem, except that my dwarves can see him through the fortifications I stupidly carved. Also, he keeps shooting web through said fortifications. This would STILL not be a problem because my dwarves rarely go down that far and every time previous he came up near the staircase he eventually swam away, BUT I just had a battle down in the next cavern level against a fire-breather that wrecked all the shit. There are nineteen dead dwarves and the surviving loot down there, and of course everyone wants the ☼pond turtle biscuits☼.
Obviously the solution is to burrow everyone away and wait for him to leave again, hoping that the ghosts don't rise up and traumatize everyone (more) before I can get this dealt with, then blocking up the fortifications, but I am hoping for a more... permanent solution. Simply walling him off is too good for the level of annoyance this fucker has caused over the five-odd years he's been dicking around down there. So many bolts have been wasted on this asshole, there are over nine pages of them sticking out of him and at LEAST two more in the water. Just before things got really bad this last time I had over 5000, and now I'm down to 3600 (obviously some went into the firebreather, but still). Literally every part of his body is broken and cut open, and has been for some time. Clearly dying isn't in his calendar planner, but he needs to.
However the only way to kill him seems to be either tempting him onto land somehow, which I'm not sure he even does, or doing something really dwarfy. If you have any suggestions, I'm all ears.