The Neo-Soviet marine biomods
Classified as a category 4 threat, the Elib is somewhere between a vampire lord and Godzilla for limits of escalation. Yes, I would consider using a nuclear device to kill just one. I am that sure that I want it dead.
With regard to the plot "Sleeping Mines of Elathon":
You manage to get the water wheel in the mine working which operates a pump to clear the flooded lower levels. As you explore the dank tunnels of the lower galleries, you can hear the creaking of the wheel above. You enter a low-ceilinged passage and are crawling on your backs, when the creaking stops. The water starts seeping wetly around the back of your head...