Posts from How to deal with Lethos’s plague (Dark Elf)?
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.
И пока в кармане пачка милкиуэйев - значит все не так уж плохо на сегодняшний день
luck - soldier's luck, magic resistance, elven luck.
If you think about it, you can take Darkness as the fifth skill: the map is large, so there's a high probability of finding the necessary spells, and then not only Letos will be throwing berserks/subordinations, but he will also be very surprised when he opens the spellbook and sees two red inscriptions there - Seal of Darkness and Weakness of Darkness.
Wing is considered one of the overpowered heroes.
Did you say that?
И пока в кармане пачка милкиуэйев - значит все не так уж плохо на сегодняшний день
And also, Necromancers.
Don't say that, Necromancers aren't cheaters, they just dominate on most maps.
or we accept the game balance as it is.
Generally, Fire Elementals are stronger than Warlocks; I'll post a build against Leathos soon.

The idea is to suppress the plague from the very beginning, and to prevent him from casting Darkness on you. You can also upgrade Blessing and Seal of Darkness to "Seal" the ailment.