Did he use any cavalry? If he brings desert cavalry, you can't beat him with a simple heavy cavalry flanking manouvre. Try using VERY strong, upgraded phalanx footmen and charge his front. If the footmen are winning over his footmen he might feel forced to try and flank you. If not, you can, when his footmen are pinned, bring chariots and/or elephants and cavalry around the flank to kill his archers and cavalry.