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Armageddon can be easily countered, and its damage isn't that high. An explosion deals more damage. Even with a large group and speed boost, no explosion will seriously harm them. As an alternative, use good archers combined with general slowing effects, force fields, clones, antimagic, and teleports, in case they get close. But again, it's unlikely to work – the necromancer hero will probably have counter-measures and a decent army by that time.
