The essence of the tactics when playing as the Dark Elves.

It's strange, but Letos doesn't seem like such an overpowered hero to me. At least, in theory, it sounds good, but in practice, it often turns out the opposite. Firstly, he desperately lacks mana.

Letos actually has plenty of mana. Think about it - even with a basic Spell Power (SP) of 3, as Fireball already mentioned, we get 56 damage from Plague. Over 3 turns of invisibility for the Hunters, the enemy will take 336 damage with an initiative of 10. Now imagine the enemy is, say, some Wind Dancers with an initiative of 15, high defense, and good morale. No archers or machine gunners will avoid losses.

It turns out that Plague can only be cast once, and its ability triggers very rarely, and I don't understand why he's even better than machine gunners at clearing out enemies...

The ability doesn't trigger that rarely. And he's better than machine gunners for one simple reason - he's a Warlock. And therefore, he has Hunters. Machine gunners don't have Hunters, which means there will be some losses, even if small. In particular, this is especially evident in battles against Druids and Archmages.

And why does he need Dark Magic, since it requires a lot of mana and is designed for those who have problems with spellcasting, but the Terran Enchanter has the opposite problem - he lacks mana.

Low-level Dark Magic doesn't really depend on SP (except for Plague), but in high-level magic, Vampirism, Dominate, and Berserk show their immense power, especially for spellcasting races. For example, the Necromancer. The problem with the Terran Enchanter is how to get these spells. But even if he doesn't get them, there's always Slow, Forgetfulness, etc. Slow down, and then crush them with Chaos or Summoning (whatever he has leveled up)...

Explain in more detail why specifically Kifrah.

Well, on short maps, she can disrupt a machine gun rush due to her "built-in" Stone Skin. Fire Warriors won't be able to handle it, because the machine gunner will have a Ballista with 1100 HP. That's a lot of Elementals needed.
And on medium and long maps... We recently developed a build for Kifrah specifically as a power hero for medium and long games. In practice, we haven't tried it yet, but it might turn out to be a deadly combination. So, the minimum set of skills:
Leadership up to Empathy
Logistics and Surprise Attack (it's a pity you can't get Keen Mind with her)
Education with Magic Attraction (Scholar + Keeper of Secrets for the same Empathy)
Attack up to Haste
Spell Power.
And Elemental Sense is welcome.
The battle goes like this - the troops are accelerated, and with their morale and initiative, the time between casts is significantly reduced. Then, Kifrah just teleports Minotaurs, moving them up the turn order. And under her, these "bulls" just go wild. (As an alternative - Riders, so they can "ride" along the entire enemy army)
I'll try it in action - I'll write about it...