I've seen stuff like that tagged as 'canon-typical violence' before, which seems to work.

I haven't got anything against the "author chooses not to use archive warnings" tag. It's honestly a great system and I feel like it does its job quite well. It's only when people decide to get all wanky about lack of readers/involvement that I get annoyed, because the "author chooses not to use archive warnings" will, by its nature, mean that some people avoid the fic.