Tactics for Necropolis.

The most useful spells are mass slow and raise dead.
As for building up vampires quickly, I disagree. Take the hero Tanta, and he can clear the territory just as well as vampires (with his starting army)... and by the third week, while others are still building up vampires in Vampire Lords, with Tanta and 500-700 skeletons, you can already attack Utopia.
to Qwerty (it's difficult and not necessary to build vampires with a castle on the first week) 2-4 vampires won't make a difference... it's better to boost the economy or build liches if you can find sulfur (10)... the exception is if there is a real threat to the castle at the end of the first week, and the main hero is far away (because if he is close, no one will attack Tanta on the first week).

Added 1 minute ago

It's better not to learn water for a warrior. Teleport works without water, you can also clone skeletons, and prayer is not learned in Necropolis, so it is learned quite rarely.
And about the explosion: why waste a spell to deal minimal damage to the enemy? (he's a warrior, after all)

Water is really not needed... the main thing is to take earth, and then either air or fire, and I generally try to take wisdom as the 6th or 7th skill.