From Longman Dictionary of Contemporary Englishgang up on/against somebody phrasal verbCRUELif people gang up on someone, they join together to attack, criticize, or oppose them, especially in a way that seems unfair Schoolchildren are quick to gang up on anyone who looks or behaves differently. → gang→ See Verb table
Examples from the Corpus
gang up on/against • Baboons will gang up on a leopard in a similar way, although this is a risky venture.• Schoolboys are very conventional and quick to gang up on any boy who in behaviour or dress does not conform.• The Harpies are vicious, hungry creatures who will gang up on any isolated individual.• You can choose which individual member fights the monster, but forget about ganging up on him.• Fancy that lot ganging up on me.• Now, however, there are worrying signs that the two biggest firms are ganging up on the rest.• They're all ganging up on you, the banks, the tax people - and you've got to survive.• Sometimes friends gang up on you. 