Once you start doing more fights it's too slow to do all but last fights manually. The game changes quite a lot as u progress along story questline, don't need to worry about losing units or material costs when negotiating. In the endgame the limiting factors for doing battlefield fights in particular are attack and defense, my bias is in favor of more attack, it takes a lot of defense to make a difference. When u can c the units u r going to fight it gets pretty easy to remember when the game AI isn't very "I," I always do those battles manually.