Basicly, no matter which mage you play (except for another necromancer), a necromancer must be rushed since otherwise he become too strong to stop (especially zombie necromancers). Luckily, necromancers have some difficulties standing against a rush, as he is the only 32 life mage that doesn't have a special ability that grant's him defensive capabilities (a pristess gain life while healing, a wizard have voltaric shield and forcemaster have deflect).
Rushing a necromancer that plays zombies can be a bit of a problem. I played a zombie build against my mate, who was the forcemaster. He rushed, and I took some damage, but I had enough tricks to hold him back, armour, brace yourself, etc. and I just kept dropping crawlers, and he couldn't cope with the 3 attack dice from each. eventually I got out a brute to guard and in the end the zombies pulled his forcemaster into meaty chunks. Haha.
But yes you are right the necromancer cannot be left as he becomes very hard to beat if left alone. Its all about striking the right balance.