If you want a single, collective noun to refer to a group of bullies , the word 'mob' could have this connotation. Dictionary definition: Mob: a large crowd of people, especially one that is disorderly and intent on causing trouble or violence.

I would use the word "gang". This word is used in newspapers to describe "gang rape", "gang-related killings" and "gang violence".

You could use a phrasal verb "to gang up on someone".

Bullies, thugs, cowards

That's an interesting one:) The first word that came to my mind was "bullies" but it's not the word you are looking for I guess. If we knew the context, it might be easier to help you