First of all welcome to the game :)
Second, I think you will find it hard to keep casualities below 8%, i did. The most rewarding way to attack is to first bombard with your artillery, then you aircraft (or is it the other way around), and then, to hit from multiple sides when you attack.
Generally you will find the losses you incur to be higher on the first turn, because units have had time to entrench. Bombarding with artillery and air will reduce that entrenchment level. Also, units that have fled have no entrenchment bonuses, so it might be a good idea to follow up attacks, when you already have made them flee on the same turn.
Use you mobile troops to surround the enemy and make "pockets" of troops. These will be out of supply and when they don´t have any more supply themselves, unable to move. Units that have nowhere to flee are also destroyed. It is worth much more to destroy units, than just make them flee and fight another day.
I wish you lots of fun in the game.