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In any case, antimagic will be preferable. Because the mirror won't reflect Mass Slow and so on. It makes sense to make the mirror a mass-magic spell. And in general, I think that rebalancing magic deserves a separate topic.
It should be done so that the mirror reflects mass magic, including expert dispel. The point is that with air magic, the mirror will look no worse than antimagic - you can revive, teleport, etc., but casting Mass Explosion on a mirrored unit is not ideal. This is especially relevant for battles where a stack of ducks is facing a stack of skeletons, for example, and the rest of the troops are just there.
Пусть мой враг будет силен и страшен. Если я поборю его, то не буду чувствовать стыда.