there are certain parts that are more difficult than others - I find that the place right after the palace district, at the bottom of fort drakon, is where I need the most help. I never use any of my armies before that. and just to let you know, if your persuasion is high enough, after the deep roads/dwarves quest, you can talk the legion of the dead guys to help you out. they don't show up in the army picker, they just randomly show up in places.