While you can't control what units are in a battle, you do have a limited ability to control the order of units that enter the battle from your party (or your parties in an army).
The units you bring towards the top in the party screen seem to be prioritised in battle.
So for example, if you bring your archers to the top, you'll have more archers in the battle. So I tend to organise my own party, and other parties in my clan in a way that offers me superiority in the area that the enemy I face is. E.g. I bring my archers to the top when fighting Battania - who usually only have a handful of Fians. I bring my heavy cavalry to the top when fighting Khuzaits who's light horse archers can be minced by heavy cavalry. Etc. Etc.
It isn't 100%, but it does work.
Otherwise, you can just transfer units to an empty selection group and withdraw them if the need arises - E.g. protect your Tier 6 infantry by moving them to the heavy infantry slot, and withdrawing them from the battle.