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I don't like Nazir, except maybe for using him with a "hithran" strategy. Otherwise, he's too specialized. Among fire spells, there's only Fireball and Armageddon (the latter is what we use for the "hithran" strategy, combined with buffing gargoyles and alt-buffing golems). But you need to consider that we might not get strong fire spells in the city; it all depends on the random chance of what appears in the library. If there's a spell library on the map, then yes, you can use it, but again, it's better for the "hithran" strategy. If "hithran" is prohibited, I wouldn't use it at all.
Yeah, Nazir will also lose effectiveness in battles against fire elementals, dwarven dragons, and those who have immunity to any type of magic or have more than 50% magic resistance.
Overall, some units from the Academy are specifically adapted to resist magic, so you can deploy obsidian gargoyles and obsidian golems and start using Armageddon on everyone. :)