I also couldn't defeat Steadwick in the last mission, but I did something quite simple: I bought a lot of heroes (there was a tavern nearby, I think), sent them near the castle (some a little closer, others a little further away), and when the computer's turn came, it simply went to eliminate them, and then I just entered the castle with another hero.
I also couldn't defeat Steadwick in the last mission, but I did something quite simple: I bought a lot of heroes (there was a tavern nearby, I think), sent them near the castle (some a little closer, others a little further away), and when the computer's turn came, it simply went to eliminate them, and then I just entered the castle with another hero.