I would say that versus monsters in later stages of the game (barraging River Of Flames on Hell difficulty to level) necromancer becomes the easiest class to play. You summon a Fire Golem, and it will kill a monster or 2. Immediately revive them. This will spawn some tanks having 2-3x hp of the normal monsters (depending on the skills). They you just cast Iron Maiden on the monsters and watch as your revives hack&smash through the crowd. Add more revivis if needed.
If a monster breaks through your line of revives, kill it quickly with a bone spirit. Do not let it attack and hit you, because their attack rating is decent enough to score every hit, and the damage will soon make you able to cast Corpse Explosion on yourself
A large crowd of summons is a barrier only bosses will be able to break, and still such of them as Duriel fall after 1-2 attacks of their own - thanks to the Iron Maiden!
Try it out yourself, Necromancer IS a class worth playing!