An easy way to kill off enemy armies in battle mode is to bring your infantry to the bottom of the screen, move backwards to get them into retreat formation, then move forward to the enemy. You will see that the enemy can't get quite as far down the screen as you and so they are stuck trying to move downwards. Now you can just advance and shoot them all down with minimal losses. This will only work when playing the computer of course. It may be harder to win if your human opponent has read this hint as well!