I'd say a destroying a city definitely meets that definition. Deliberately destroying part of a national group. Also, the inhabitants of a city are definitely an ethnicity (though that depends on how restrictive a definition of ethnicity you use.) So, sure, you kill a lot of people it's not genocide, but you systematically eradicate a city, I'd call it genocide.
The inhabitants of a city are not limited to a single ethnicity, so it's still fundamentally wrong. However, this exchange won't get us any further. I have no doubt you won't be convinced, so there seems nothing to be gained by continuing. Feel free to use language as you will.
> pertaining to or characteristic of a people, especially a group (ethnic group) sharing a common and distinctive culture, religion, language, or the like.
The inhabitants of a city usually share a common and distinctive culture (and the like.) You're using extremely narrow definitions of genocide as well as ethnicity. Both are vague terms.
