samadura
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I usually played against Necropolis with two heroes. The first hero would get undead troops; his main task was to eliminate the vampires, and then the second hero, with a strong attack force, would attack.
Losing vampires, even in a won battle, demoralizes any Necromancer.
And regarding your tactic described above, I'll say this: you probably used this tactic against the computer (something tells me that's exactly what you did);) because gathering a second hero with specific troops for a particular opponent... it's not something that happens in games against human opponents!
Even if you managed to do it... you caught the Necromancer hero... but a person playing against you will never let you easily defeat the vampires... they will protect them, and if necessary, raise them with magic.