It is dangerous to use Large scale magic in the early stage of the battle. In large-scale battles like this, the magician will become a primary target for everyone.
Although it is good as bait, it will also make the casting sixth-grade magician useless for the rest of the battle whether they successfully cast the spell. Moreover, there are a lot of combatants with full strength on the Nergal side to interrupt the duchess's casting.
No matter how thick and durable her protection, she will never be able to withstand the continued attack from the Nergal's army, especially from their magician unit. Magicians are famous for their firepower and attack range.
Some spells are even designed for long-range attacks. Moreover, the fourth-grade magician could use Flying spells and maneuver in the sky while casting their spells.
