Wiktionary
gang up on
vb. (context transitive idiomatic English) To join together in a group in order to overpower someone else.
Usage examples of "gang up on".
As the furious Ewoks gang up on him, the Lahsbee indeed Hukhs out - and goes after a fleeing Hiros.
Your system is a legal civil war, where men gang up on one another and struggle for possession of the law, which they use as a club over rivals, till another gang wrests it from their clutch and clubs them with it in their turn, all of them clamoring protestations of service to an unnamed public's unspecified good.
We have to keep our distance from the main group, though, or they'll just gang up on us and eat us all.