Well, the battle with the Necromancer wasn't too easy, but I won, rebuilt the castle a bit, gathered an army, and went for the purple faction, while I had another hero gathering an army from the main castle. When Sigrif defeated the purple faction, I barely had an army left, but Jezabeth with her army saved the day.
I played this mission using a strong defense and the "Devil's Chain" spell. I took the "Cold" reward and "Distraction" from the magic guild, plus +2 to spells and levels 5-7, which gave me a lot of spells in total. I ended up defeating the orcs like that, and in the end, I fought the Necromancer with 20 spells; the "Devil's Chain" spell one-shotted everyone:) By the end of the mission, I had 24 spells.
Added 1 minute ago
I just think this mission can be completed without problems only in this way; using machines is silly, and the attack is also not effective. Oh, and yes, I found boots that give +1 to all parameters and a robe of Sar Iss.