To keep your units in formation and prevent them running all over the place in pursuit, group them in formation and put the group in Guard mode. That way they will emphasise defense and keeping formation. Use that to pin the enemy then as he tires you send in the cavalry you have sent around his flanks, charging him from behind and/or change out of Guard mode and attack. This keeps formation for all factions, but for the Phalanx ones it is essential.