Ganseng
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Well, if you let the battles drag on to the 20th or 30th weekly reinforcement, then of course it's a problem. And that's why you need to eliminate the Necromancer starting from the first month. And since I hardly play on maps larger than average (first, it takes too long, second, the game starts to lag), this is quite achievable. Again, I repeat, IF you're lucky and get a good starting position, then of course you go for Haven. But that's probably a one in 20 chance. And besides, where will the demon learn light magic if there's no neutral city like Haven, Sylvan, Dwarves, or Academy? So you might end up with neither a holy weapon nor even teleport, if you don't get the "surprise attack" skill.
And Necropolis is far from the strongest faction. Since Heroes 3, they seem to have been strengthened with abilities, but their overall stats aren't that high, with average defense and almost zero attack, especially for dragons, which frankly aren't worth the resources spent on them. It's better to use those 20 gold and 20 mercury, which you would spend on them, to upgrade vampires and undead or build a level 5 guild of mages; you'll get much more benefit from that.
And Necropolis is far from the strongest faction. Since Heroes 3, they seem to have been strengthened with abilities, but their overall stats aren't that high, with average defense and almost zero attack, especially for dragons, which frankly aren't worth the resources spent on them. It's better to use those 20 gold and 20 mercury, which you would spend on them, to upgrade vampires and undead or build a level 5 guild of mages; you'll get much more benefit from that.