The main spell of the Necromancer is to summon minions.
You can summon a max of 5 at once once you have gathered enough life force from killing enemies. Your best bet is to find a location with a lot of spawning enemies with multiple summoning Stones.
Earn enough life force to spawn 5, find a nice spot where you can funnel the enemies and summon the 5 minions. As they kill you will gain more life force and if you attack with them you should gain enough life force for another 5. Keep repeating until you reach 1,200. Ghostwalking accumulates life force as well, ABUSE IT!
You will need to spawn 5 minions 240 times to achieve this. I used the