Tactics against Necromancers.

Humans have a significant downside — army speed (except for archangels). That is, if there is no General slow, the archangels move first, but then it's the undead's turn (which includes dragons, smart knights, and vampires). Therefore, one shouldn't rush into the decisive battle too quickly. On the other hand, the Tower and Conflux have an advantage in that vampires do not resurrect from them. So, when playing against the undead, this must also be taken into account.

I disagree with you regarding army speed; what about champions? Perhaps only phoenixes are faster than them. From there, everything increases geometrically: we move first (especially if our hero initially has the +2 army speed ability), cast General haste, and at least 2 of our units reach the enemy. And we already know how champions hit when they have a full run-up, not to mention the Archangels!
Furthermore, everything depends on the magic used by the enemy.