Who says General dont learn knew things on the field. Africanus was only able to achieve that title by learning from battles against hannibal. He took what he learned against Hannibal and was only able to defeat Hannibal due to this. Although since it is really us controlling the General and his army, gaining higher attack values and defense can simulate better drills and training from the general, which he probably wouldnt learn much new things in the field. In terms of gameplay, Exponential would be the best solution IMO. Make it so, your punished for bringing a completely raw general to the field, but if they've fought some rebels, they will have some stars so the difference in rating isnt too bad between a high star generals and your general who has only practised against Rebels.