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

for me, it's not just a mission, it's a real problem. I've replayed it three times, and it's starting to get boring. I manage to capture two castles, and destroy Isabella and her elven army (but only about 50 halberdiers and 1 inquisitor remain from her army :D). Then, I start getting crushed by mages with their infernal army.
Warrior difficulty is the lowest difficulty in these Heroes games.
By the way, there are no suggestions D: