Frida's Dilemma - "Duncan" [Campaign 1, Mission 3]

Well, last time I had (meaning Duncan's army): 5 core griffons, 5 paladins, 6 monks, 10 archers, 44 armored infantry (the most annoying), and 22 militia.
Before the battle with him, you need to capture one castle (the one in the lower left corner of the map) and develop magic to the second level. Then everything will work out.