Gunslinger
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Even if the chance of obtaining light magic is very low, light magic still has the following advantages in this case: First, and one of the coolest spells against the Stronghold, is HOLY WORD... you see, it removes their rage. Second, you can significantly strengthen your troops, and against neutral creatures with resurrection (if you find them), it becomes simply excellent with minimal losses. Third, a Barbarian can deal with the Guardians! I believe that if you play for a long time, it will eventually appear; at least, it did for me. If you lack spells, play on the map generator; there are many libraries and castles, and there's also a chance of it appearing in the guild as a tome of light! There is also such a cheat spell as Divine Retribution.
P.S. I don't encounter light magic very rarely... I don't know about you, but I tried it, and I really liked the result!