Personally, after peacefully eliminating all the computer players (except Isabel), I focused on building castles, accumulating money, and developing my heroes. I cleared all the Marletto towers and arenas, and other locations, leveled up everyone as much as I could, killing neutrals and collecting chests. Then, with Godric, I gathered all the troops, upgraded the peasants to crossbowmen, and, using him as my least developed hero, defeated Isabel.
This is the second-to-last mission, and I preferred to take my time and level up.