I think the most bang for a rogue nation's buck when it comes to devastation would be to fire of a couple of nukes to create a lahar on the Cumbre Vieja of the Island of La Palma in the Canary Island. The ensuing tsunami would pretty much wipe out all the coastal cities of West Africa, Europe and North America.
Heck most medium sized nations with nuclear capability would have a hard time beating that devastation. And I don't believe it would take too much ingenuity to pull such a thing off.