After capturing the first Elven castle, I bought another Elven hero and, after each turn, transferred Elven troops to him, as they flee from Agrael. Next, I didn't research anything and immediately captured the Demon castle, quickly built it up as much as possible, recruited creatures, and, after killing the dragons, rushed to the capital. I don't remember exactly how much time passed, but I remember it was very little. I couldn't fully level up Agrael in the campaign, but he wasn't there at the end of the story anyway (in the sixth campaign). I played on Veteran difficulty.