All of the ones that targets troops in a province
Rain of Toads, Leprosy, Melancholia, Fires from Afar, Flames from the Sky, Muredering Winter are especially bad on grouped armies and regular assasination spells are options too of course.
On the other hand Honey, if all grouped in one province you could more easily protect them with a dome, late game that might be the better option (less provinces dying, if you have several large stacks you'll might want to protect them all anyway, possibly less microing.)
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