How to deal with Lethos’s plague (Dark Elf)?

luck - soldier's luck, magic resistance, elven luck.

I wouldn't take luck without СА (I don't like relying on randomness), but it's not up to us to decide)
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.