I don't know what their action will be, but an artillery attack will force the South Koreans to invade in order to silence the guns. The SKAF will have air superiority very early on.
I think a (suicide) commando operation is more likely, because I don't see how you kan kill someone with a cyber attack. Maybe they already have a 5th column inside SK.
Anyway, an attack that kills the political leader(s) of SK means war. NK is playing with fire.