If you play as Orcs against Necropolis, you need to level up Leadership, Tactics, and Speed. Also, find boots that increase speed and artifacts that increase Initiative. The idea is to eliminate the Necropolis troops faster than he can mess things up with dark magic. If you approach him directly, it's risky to use Berserk on the Necromancer, as his troops might damage your own. You definitely need to learn the Darkness/Summoning spell and the Magic Resistance skill – the Necromancer will cast spells less often because the mana cost will increase, and he will be placed at the end of the turn order.