The snowballs will humiliate them - they will make them cold and wet and unhappy. Is not humiliation a proper form of retribution?

[Not that she thinks it'll be humiliating, but it will be something that won't see her friend physically harmed or harming another.]

Not all who choose to fight in small ways are 'easy pickings' as you say. There is strength in cunning.

Anyone who would consider you weak is a fool, Vriska.