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No, 1000 is not possible. Only if it's a player, and they have no more than 3 stacks, all clumped together. This is practically impossible, so there's no point in considering such a situation.
The AI, when facing a single Demon, will split the Archangels into 7 stacks.
However, a smaller number is possible in the following cases:
1. It must be in a castle, and you must use tactics so that the turrets fire first. In all other cases, the Archangels will move first.
2. There must be a VERY strong mage, with expert Earth magic and the Summon Elemental spell (preferably also Earth).
Summon elements from the first turn; there should be enough of them so that the Archangels don't kill them in one turn. The Demon will naturally be killed; increase the number of elements with each turn, and the number of Archangels will decrease.
Before killing the last Archangel, resurrect the Demon (because otherwise, the elements will disappear, and the defending hero will also die).
The tactic is simple; I think many people have used it. I wrote this for those who are not aware.
This is how I used to complete the campaign.