First I am going to saw that the necros base spell book is one of the most strait forward book to run- its (And this is a technical term) RARW ZOMBIES!!!
But seriously, I have noticed that the base necro book is a good one, and its hard to go wrong when playing it.
For the overall necro- Yes, he is powerful- but in my many, many matchups with him, I have found many tricks that shut him down.
First off- he is stretched thin for mana- those hoards dont cast themselves! Since the necro spends most of his mana (And actions) summoning and getting out his spawnpoints; in the early game, he is offten left unequiped, and even twords the late game, he is usually very unenchanted when compared to most mages. And this is not a side effect of play style, its something that comes from swarming with the necros medium costed creatures. (You can supper equip/enchant the necro just fine- but his swarms will suffer)
Another thing is that the necro can really easly be "The odd man out" or targeted easily. Its not hard to separate him from his creatures. And I would say that the best chance that a forcemaster would have it to push everything else away, and keep the necro held with something like force crush.
Cast tangle vine, teleport, force crush, ANYTHING to keep the necro away from his creatures. A necro with Alter of the Iron Guard and zombies poping up all around him is a VERY tough opponent indeed.
Another way that he is easily targeted is by a Wizard. IMO the wizard is the perfect Necro counter. He can just blast spells from afar and keep pressure on the necro- causing the necro to change his plans and really mess with his strategy. Wizards tower can even flank him, causing his zombies to slowly shamble in different directions, which makes them much easer to pick off.
And with the beastmaster- He can enchant his creatures; the necro cant. Buff a bear with Bear Strenth and lions savagery, eagle wings and maybe a defence and have it hover over the necro for the game, and even with skeletal archers and attack spells- you should have one dead necro on your hands.
TL;DR- the necros creatures are slow (Lumbering!) if they are resilient and he can be easily "Sniped" by mages with attck spells.
The necro focuses so much time and mana of getting those swarms; he often leaves himself unequipped and unenchanted.
The necro cannot enchant his undead creatures.
Really its hard to confront the necro head on- but with some range, and lots of tricks, he should be defeated with a little effort.