Swift focus attack. Since both side has equal quality of troops and terrian is simply mundane, that what you do.

Which every side, flank, or center come into contact first, crush it with the fullest of your troops and continue to do so to the others. If your enemy remains in active and split his troops as shown, take your own initiative and hit the weakest part, crush them one by one.

It is a simple theory, when you enemy is equal in strength, do not split your force.