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Against any living creature, any Necromancer is suitable. My choice is Zoltan because of his ability to block the opponent's spells and his starting education with the ability to solve a secret puzzle at the beginning.
Against his own kind and Razzak from the Academy, Orson is the best option.
P.S. I believe that Necromancers are the strongest heroes in the game. Only mages from the Academy can compete with them (provided they focus on obsidian golems and gargoyles).