Hmm... Usually, I don't focus on upgrading units right away in my tactics, prioritizing building all dwellings as quickly as possible, but it seems that with a Dark faction hero, it's better to build two full upgrades first, then Hydras, and only then, after securing resources, build the rest.
By the way, I also started with Vaishan, but the lucky wizard only appeared at the end of the game =D

I used to do the same – build all the growth structures first, and then upgrade, but I realized that it's not efficient. You might gain a bit more growth, but you'll lose more by fighting with un-upgraded units in the meantime)
The randomness in Heroes of Might and Magic V is insane! In the demon scenario, on the third level, I found a very rare light skill for him, but in the warlock scenario, Chaos, which is one of the most frequently offered skills, I only found it on the eighteenth level).