Then, let's say, give Knights the "Curse" attack, and the second one the attack that Plague Zombies have.
Give Necromancers one attack with "Sorrow" + "Blindness" attack. The second one - "Fear" attack + ? (maybe some kind of aura)
Give one an enhanced "Hand of Death" or something like that - instead of killing 1 unit, let it just deal damage; the second one - enhance the damage from the area-of-effect ability. Then we won't give them any spells at all, but give Knights "Dispel" and "Blindness," for example.
Added after 57 seconds
And mummies instead of zombies - just for good measure ;)