I answered a similar question for Ancient Greece here. The main thing is that we shouldn't be distracted by the absurd lethality of pitched battle in movies. That stuff is just for show. In reality (both in Antiquity and Medieval Europe), encounters between heavy infantry would likely be far more tentative, with few casualties, until one side broke and fled. The great majority of deaths would come during this final phase of the battle, when one side had ceased to offer organised resistance and the other side pursued and killed with impunity.