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

On a specific map, like when testing on the Anniversary map, yes. But in general, no, not always. Kaspar, for example, easily defeats Letos on the "Former Friends" map, and not just because of the undead's immunity to plague. Any skilled player can get a decent Utopia on week 2 without using advanced artifacts, while Letos often fails.

I disagree with this example. I played on "Former Friends" and also against Kaspar. I cleared the entire center on week 2 without any problems.
Getting Utopia with Letos is very simple – you just need to focus on developing the Summoning skill and either Mines or Crystal (the latter is preferable).

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With Crystal, there's a real bug in the AI's behavior. It's as if this spell was specifically created for Necromancers. If there are no enemies on the battlefield (Hunters are in stealth), the AI (you can't really call it intelligence) considers Crystal guilty of all mortal sins and attacks it. This is easy to exploit – place Crystal next to a stack that moves later (and in the case of large creatures, even next to two stacks), and the stack that moves earlier will destroy itself and its allies. Moreover, Black Dragons take full damage, despite the magical type of damage from Crystal.
And Crystal's damage is pretty good. And Earth Lord also affects it...