First post so go easy on me.
I am in love with the Necromancer, but I do not play him swarm style, You psych your opponent into believing you are about to go swarm but end up setting up a small fortress, and then going solo to defeat your opponent.
Something that always appealed to me when playing the Necromancer was his way of getting Life Bond on the alter of skulls when finite life was activated. Keep a Mort with his Reconstruct 2 trait for all skeletal objects in his zone and you can essentially heal the Necromancer 2 damage every turn while the opponent cannot heal and is constantly being damages 2 (3 if you drop an Idol of Pestilence)
My Idea to do this was to put an alter of skulls in a corner and keep an on guard Mort combined with an eternal skeletal archer and your cleric in the corner and then throw up a wall of bones boxing them in. Your cleric will feed your alter of skulls while Mort kept your walls from coming down and your archer could handle any flyers while being healed by Mort. Now the opponent may find a way to get through to the alter, but if you play your cards right and put up good defenses like teleport traps, poison gas clouds, hell fire traps, caltrops, and maybe Malacunda as a moat to your wall of bones corner, then your opponent is going to have a hard time bringing down the alter of skulls.
Once you have that set up, you can go totally solo Nec with your cloak of shadows, deathshroud staff, demon hide armor with some other form of armor combined with veteran belt, cheetah speed, and a defense or two (I also highly recommend an elemental wand with acid ball to take care of any tanks). You do this and you are a big threat. Your opponent will be trying to get through to your Alter while you go after his force. The Nec makes a difficult opponent to kill when he can take almost all blows, heal 2 damage a turn, has poison immunity, and give constant Rot conditions that he can make extra painful.
While your opponent might eventually take out your alter (provided you play smart and keep it defended and throw up more walls when they go down) you will have devastated his health and his army leading to a ruthless victory.