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After a couple of unsuccessful attempts to complete this mission, I realized that the first thing to do is rush to the castle. But after clearing the garrison, don't immediately run to the Necromancer holed up in the fortress; instead, wait nearby until he comes out on his own (he came out right away for me). At that point, the castle walls won't be an obstacle. Then, you need to build a Guild of Mages to level 5, and then run around the map, gaining levels in battles to reach level 10. I chose skills that are useful for spellcasting (they don't give me any attack skills...), and also luck. I had to give up on logistics...