SAF
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To be honest, I don't understand your concern at all. Given the conditions you described, you shouldn't just win, you should completely crush Leto. There's no need to worry about the plague; by level 30, you should have a sizable army, and the plague deals very little damage (even if Leto's spellcasting is at level 30, it's only 272 damage over 4 turns). Besides, you have Wing, so half of his army will be eliminated before they even get a chance to move.
Regarding upgrades, I definitely recommend focusing on light magic, logistics, attack, and defense. You'll have one free slot left, and I would skip special abilities (although if you have a mentor, you could take it) and instead choose leadership with a speed aura and retribution, which guarantees that most of your units will attack on the first turn and deal massive damage thanks to retribution.
In short, the outcome of the battle should be decided in the very first turns, and when Leto's turn comes, he will only have remnants of his army left, and no berserkers or mind control will save the situation, and they can be dispelled with light magic.
Regarding upgrades, I definitely recommend focusing on light magic, logistics, attack, and defense. You'll have one free slot left, and I would skip special abilities (although if you have a mentor, you could take it) and instead choose leadership with a speed aura and retribution, which guarantees that most of your units will attack on the first turn and deal massive damage thanks to retribution.
In short, the outcome of the battle should be decided in the very first turns, and when Leto's turn comes, he will only have remnants of his army left, and no berserkers or mind control will save the situation, and they can be dispelled with light magic.