“Elia’s Quest” – “The Showdown” [Campaign 3, Mission 5]

I'm having trouble with demons. The thing is, when I capture a Castle with Ilay and Isabel (Isabel just stands around and waits), a demon appears with about 14 Archdevils. But that's not the real problem. The issue is that 150 demonesses can wipe out my entire army with a single shot... What should I do?