Would an average reader know what "pusillanimous" means? That would be the kind of word I'm looking for if anyone would - it sounds more distant than "cowardly". "Cowardly" sounds like you have a personal vendetta against the person/deed you're referring to. I'm trying to avoid that.

The situation is this: someone is anonymously attacking someone I know, and will not reveal him/herself or back down from attacking, and I want to find a good way to describe such activity, without sounding personally irritated. Not necessarily professional or dignified, but something similar.