Besides dark magic and a sharp mind (logistics + education), a Necromancer can, of course, take attack skills for tactical purposes, to reduce the number of troops that reach the rear, or defense to hold out longer until Battle Potion is available.
LordViRuS, watch the replay and tell me if mines would have helped the Necromancer in this situation...
But a sharp mind combined with control would have helped... Of course, the armies aren't equal, but the principle is important... Orcs are capable of defeating even larger stacks...