This is literally the same problem I'm having, and now I have to restart the battle and try again to keep that idiot alive.. The game is great so far, I'm in love, but sometimes the AI is really stupid. What makes me especially mad is the fact that I needed to kill just 2 more and I was done.. Well, I think I'm gonna F5 every single turn and hope for the best..
Good luck to you too fellow Godwoken