Well im pretty new here so bare with me and help a little.....I like to use guerilla warfare on the open field. It works best with the Romans. What I do is hide my archers in a wooden area with the majority of my heavy militia behind them. I rush the enemy with light troop usually light calvary and then run to the spot of ambush. I return my light troop(which are pretty battered) behind all my troops while my archers devour the chasing enemy and finish them off with my mian army.