Necromancer – “Invasion” [Campaign 3, Mission 3]

for me, it's not just a mission, it's a real problem. I'm replaying it for the third time, and it's starting to get boring. I manage to capture two castles, and then I destroy Isabella and her elven army (but only about 50 pikemen and 1 inquisitor remain from her army :D). Then, the mages start crushing me with their infernal army.
The "Warrior" difficulty is the lowest in this game.
By the way, there are no "Inspirations" available.