I'm not very good with combat scripts. I've encountered a similar problem, but I managed to work around it without finding a solution. In general, I think it's better not to change the victory conditions in battles; nothing good will come of it anyway. For example, in the developers' map (campaign 6, mission 5 – by the way, that's the best place to look at combat scripts, as there weren't any more complex scripts in the entire game), the "banishment" spell allowed you to end the battle as soon as it was your hero's turn, which is, to put it mildly, imbalanced.