Drone tanks are very difficult to use, but the Necromancer makes, say, the Overlord look easy. Tanks with high rates of fire or powerful bullets can quickly wipe out the Necromancer's drones, leaving it defenseless. And unlike the Overlord, the drones don't instantly start to regenerate.
I, however, was in awe of its massive drone army and potential for power. So, I upgraded to it. My very first action was a disaster. I sent my drones flying into the face of a storm of bullets. Needless to say, they were soon incinerated and I quickly followed the same fate. Lesson learned.
But I don't give up easily and only a few days later, I tried the Necromancer again. This time, things were different. Unlike the previous time, I was using a tried and tested build (Sauron's Successor; credits to Ursuul; USE IT).
Also, this time I was more cautious and conserving. I made sure to stay close to reinforcements (AKA more squares). If I saw a bullet-spammer, I retreated (unless of course I caught it unawares which sadly didn't happen). Even when facing a small Twin or Machine Gun, I kept close track of my Drone Count. Against one enemy, I saw the cloud of bullets hitting my drones and retreated, only to find that none were killed. I instantly backtracked and slaughtered him.
This time around, I discovered what is perhaps the Necromancer's greatest strength—defeating rammers. Just move your drones out a little, wait till the tank has caught the bait, and then attack! The cloud of ~30 drones is very hard to dodge and does massive damage. Smashers and small fast tanks alike will die.
So while the Necromancer has many weaknesses and is tough to master, it can be incredibly rewarding and may prove to be your favorite drone tank.