How can I better use archers to actually hit infantry during a fight with an enemy that is half calvary half infantry?

I tend to build mass armies of one unit, because in large battles I feel like my archers are merely shooting whatever enemy they want instead of say the pikeman that they have a bonus against. Sure I can select my archers and have them attack a single pikeman, but they will likely go back to shooting the calvary after. Is there something I am missing, or is the micro that hard?