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

Skilled Summoning + Earth Lord, Crystal, SP=7, 30 mana units.
This is enough, although more mana wouldn't hurt.

3 casts. Each crystal will deal 340*2=680 damage. Total 2040 – approximately 10 dragons. There are about 24 of them in the maximum difficulty. So, you need to make the dragons attack each other, and even then, it doesn't seem very reliable. I'll test it when I have time. If you have a ready replay, post it here. It clearly relates to the topic.