La Louvre
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I also had Railag as my most developed hero. I was simply advised by a player to attack with Shade, as she had a lot of area-of-effect spells. And in general, I attacked with all my heroes (after saving the game) and killed all of Isabel's units. Although almost everyone knew about resurrection. I think that the best combination is light and dark magic. I defeated all of Isabel's soldiers with Suggestion. Only paladins remained, and I finished them off with my own forces.