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

If you're going to use Darkness, you need an expert-level spell; 3 rounds clearly won't be enough, and you should also practice placing the Crystal strategically.

It's quite advanced, because 3 rounds is all the invisibility lasts. After that, they won't just wait around.
And placing the Crystal isn't difficult. There's nothing tricky about it. Something like this (week 3 on the "Three Rivers" map, which is why I said there's no replay for the specific scenario, because the level is already quite high...):