Mission 13 TPR - The easiest win!
In that battle mission, you get Ballistas (or Catapults, not quite sure) and we all know that machines are not living creatures... but what do living creatures need to survive? That's right, food.
Only red player (you) has Ballistas in that mission, so what you have to do is next. Just wait long enough until all of their units die from hunger. Of course, yours will die too, but Ballistas won't. In that case, you are the only player on the battlefield, so you will win the battle mission!
Oh, and you have to put Ballistas somewhere where enemy Crossbowmans can't hit them.
Use this if you are really unable to win the normal way, and I hope this will help you!
~Sir Peter