Favorite hero from Dungeon.

Hmm... Actually, slowing and weakening reduce the effectiveness of enemy troops and, therefore, increase the effectiveness of the hero:D.
Warlocks are not a race that can rely solely on Darkness, so they need a second magic as well. This is why Letos turns out to be a pure mage.

The enemy may not pay much attention to the phoenix and simply destroy the troops.

Well, well. Ignore the phoenix summoned by the Warlock. You'll encounter it in battle once, and you won't want to ignore it anymore. It will be difficult to destroy the army, because 2 invisible stacks and thick Hydras with Minotaurs are a fairly strong anchor for casting.

Moreover, it takes 2 full turns to cast a phoenix, shield, and vampirism, if using Wizardry.

Well, firstly, not all spells need to be cast. Phoenix + Vampirism is quite enough, and secondly, the enemy will be slow and weak :)

Also, mana is not infinite.

And what about Wizardry + Education? Secret Knowledge, Fluctuating Mana, Magic Attraction...