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Dude, you're just not playing the second mission correctly, especially the part where you have to play as Sigrid. Basically, the idea is this: at the beginning, generate the map as many times as possible until you get a spell from the first chest, and it's essential that the witch gives you either Sorcery, Chaos, or Enchantment (this is very important because you won't get far in this mission without these skills). In Sorcery, go for Attraction and take +2 spells there; in Enchantment, go for Secret Knowledge and take another +2 spells. And it's really important to take Chaos and develop it through Fire up to Pyromancy.
Also, an important point is that you definitely need the spell generator at the beginning because there are bears nearby that you need to kill; there's an artifact that gives +3 and a statuette that gives +1.
It's also a good idea to take Defense and grab "Last Stand" there; it will help a lot.
And the most important thing: after you clear everything out, there will be mummies standing nearby, guarding an artifact (the Phoenix Feather). You absolutely have to kill them; they'll be blown away by Devil's Strike.
Then, go after the Necromancer; I only lost 2 Imps in the fight against him.