Got an artifact called Anga's Ruby? Don it, and struggle no more.
It gives a huge boost to female fighters, so I suggest getting sprites and lake fairies, dryads (optional - good if you're facing lots of level 3 or lower creatures so you can put them to sleep), and I suggest the best ranged creatures - elves and hunters. If you decide on an all elven army, you'll get a +1 morale bonus, if you don't, go with some great unit like royal snakes, emerald green dragons, shamans, or something completely different!
Some of those fights in Death Valley are really hard, I'm talking about the fights against the dragons, of course. I left them for later, came back after I cleared most of the demon lands actually.