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

3 casts. Each crystal deals 340*2=680 damage. In total, 2040 – approximately 10 dragons. There are 24 of them at the maximum. So, you need to make the dragons attack each other, but it doesn't seem very reliable. I'll test it when I have time. If you have a ready-made replay, please share it here. It is clearly related to the topic.

There is no ready-made replay with exactly this setup. :(
Only the Black and Emerald stacks are hit by the crystal. They have 960 and 800 HP respectively. 3 Crystals – exactly 960 damage. The Twilight and Green stacks are easily plagued. Moreover, one of them is consistently plagued with Lethos' specialization. I don't know why. Furthermore, very often the crystal can be placed so that not 2, but 3 stacks take damage. That is, either the Twilight or the Green stack will receive additional damage.
And it is desirable to have level 1 Rangers who will be there, near the Dragons, if the invisibility has already ended, and the stacks have not yet died from the Plague. Then they will simply have to be finished off with a direct hero attack.

I mean Utopia 4-4-6-8.

P.S. Yes, and a Crystal with SP=7 deals 100+20*(7+4) = 320 damage...
P.P.S. And regarding making the dragons attack each other – it won't work. They do attack each other, and the log shows that damage was dealt, but they don't die. It's a bug.