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Yes, once, the path to the objective was blocked by a bunch of Chaos Hydras... I just killed all the enemies and sat there, building up my army. And then I wiped out several thousand of these hydras... I don't remember if it was with magic or not.
Magic won't help much against dragons.
But in a really difficult battle, the tactic is to use spells from different schools; I don't even know which ones.