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.
The Petty Kingdoms is a region consisting of hundreds of small states. Some are kingdoms, some are city states, some are large farming communities, some are duchesses and baronies and so on and so forth. Within a few miles the PCs might reach a new "country" with completely different rules and norms than the one they left behind. "What do you eat here? Humans ye say? Lets turn around fellas."