They say a summonmancer is one of the easiest ways of finishing the game. At the higher difficulties, ordinary skeletons are more efficient than Mages.
Max Raise Skeleton, Skeleton Mastery, a few points into revives, a few into Mages, few Corpse Explosion,a Clay Golem, and one point in each Curse. Add some + to all skills items and you're done. Only problem is, when playing on realms, your army is reset at every saving. In hell is waaaaaay hard(next to impossible) to kill the first monster, in order to raise a corpse, with a solo Necro. Other than that is piece of cake.
Playing in parties is a whole different world. Skeletons have trouble killing the incresed life monsters in 8 player games, and other players simply hate summonmancers, because of the lag they cause. When playing in group, the best option is to support the others by applying curses only and using Corpse Explosion.
[Edit]Forgot to mention: ideally you'll max Summon Resist also, or at least put as much points you can into it.