Tactics against Necromancers.

Maybe I'm missing something... What slowdown and preventing vampires from reaching archers are you talking about? Liches shoot in a cloud, and if you protect your archers with at least two groups of creatures, your losses will be significant. And if you try to block the liches with the remaining groups (maximum 4), you'll be completely crushed. So, in my opinion, speed up and attack! (All of the above does not apply to cases where you have a lot of undead units).