[13lackGu4rd]

mass destruction spells such as Rain of Stones, Acid Rain, etc will devastate communions of weak mages. so that's the standard counter. however such communions also have ways to defend themselves, mainly if they're on the defensive(hence acting first) so really it depends on the situation, which mages are communioned, are they alone or with an army(and which army), what tools you have at your disposal, etc.