NerVNotDeaD, it seems like everything is correct, and maybe you can also get the Cold skill for your Castle. I think it's a good time to attack the Necropolis around the third or fourth week.

I wonder, does anyone know what will happen if the Necromancer doesn't take the Elemental skill, but the Elf does? And then, after the Elf summons the elementals, the Necromancer also summons them. What will happen then? Will there be two elementals on the Elf's side? Or will their own disappear?