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

If the hero is Wing, and he's level 30, then a Dark Elf definitely won't be a problem for him. As SAF wrote, by that time, the plague won't do any damage anymore; he'll be able to cast Berserk only 1-2 times, if at all. You just need to level him up like this: attack - Battle Fury, tactics, retribution, leadership - rally, aura of speed, elven leadership (preferably take it later), luck - soldier's luck, magic resistance, elven luck. Even light isn't necessary. The rest is up to you. The main thing is that your army moves before his hero's, and some units will even have time to move several times. When his hero's turn comes, he will have almost nothing left. This has been tested many times. You can look at the neighboring topics. Wing is considered one of the overpowered heroes.