Actually, you definitely need vampires, skeletons too, but it's better not to go against a serious enemy without black knights. Dragons aren't essential, though. Because many other very powerful units can defeat them, for example, archangels or archdevils. Take ghosts to gain magic points and move on.

Yes, dragons are generally not worth it! They're a good translation (of resources and money, in particular); only use them if you're rich and have built everything you need. Build your army around a swarm of skeletons (but I use them as cannon fodder to protect liches and dreads, because there are many of them, and they are quite resilient for level 1; you can often upgrade this spell in the Tower, but teleport is harder to find). With vampires, I usually attack the strongest units first. If it's the Tower, then I just respawn them on gremlins. With liches, in the case of the Tower, it would be better to attack gargoyles or golems (because they won't let vampires regenerate). With a dread, you can defeat djinn (it's hard to tell what they will cast). Earth magic is essential for necromancers, forgive the pun; it has all the important spells: slow, shield, stoneskin, raise dead, and, if you have enough attack power and mana, death ripple. A good hero is Tanta; if there are resurrection spells in the Tower, then Sandro is better, or heroes like Isra, Vidomina as a "harvester," Vokial, Moandar, or, a little worse, Galtaran, Tamika – I've never used her.