Also make it so that you can select your units only once in the beginning of the game so you won’t start swapping your units around in the middle of a battle. That feels like cheating. “I’ll have this set of cards in my hand and he’s going to do that to me with his cards. I’ll just swap some cards in my hand and… there we go. I can beat him easily now.”
Maybe make it so that you start with previously used unit set and if you don’t want to change them, you click accept at the unit selection bar, or wherever the accept button would be. If you want to customize your unit selection, you click the customize button and select what you want and then accept. Then you hit start the battle and start fighting and are unable to change your current unit set anymore.