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In my opinion, if Galtran survives until the 4th month (which means the map is XL), it will be impossible to defeat him in a duel with a living opponent. I'm not considering rare cases of extreme luck, like very successful utopias built near his castle. By the 4th month, Galtran will already have a level far exceeding 20, stats like 8-8-4-4 without artifacts, and an army of 3-4 thousand skeletons (not counting several hundred vampire lords, liches, and other undead). Neither implosion, "teapot," nor clone with resurrection will help here. In a game against an equal opponent, letting a necromancer survive until the middle of the second month is a guaranteed defeat.
However, if the question is whether it's possible to deal with a necromancer at all, I think Conflux has a chance, but again, no later than the middle of the 2nd month, due to faster units and a higher growth rate. I would say that Luna, Brissa, or Grindan also have a chance. But in any case, the chance of winning lies in rushing. Another race with a chance is Dungeon, but I don't know its heroes very well. If you have experience playing as Shakti, you can try, but ONLY RUSH!!!! By the end of the 2nd month, you can end the game; after that, the necromancer will have the advantage, and it will only grow.