Does anyone know which unit is better to use when employing dark magic, Specters or Vampires? Specters are good in large numbers against archers, but after the archers fire, there will be few Vampires left!

You're probably talking about a game against Sylvan. If so: if you can reduce the archers' attack, then it's definitely better to take Vampires (after all, they're a Tier 4 unit). Against Specters, there are Master Archers (although I prefer Shadow Archers). Besides, if you cast the spell "Enemy," even a single Master Archer can thin their ranks considerably (especially with Ossir; no other archer can inflict that much damage). IMHO! !

In short, take Vampires!