Without scripting....
If you are using event initiated battles, you could remove the other members from the party before starting the battle.
With Random encounters, you might be able to use the event command in the 'Troop' tab of the database.
In either case, you could also use a 'Choice' command to allow the user to select which member will fight.
To switch members in battle, assuming you want to be able to switch members on any turn, you could use the same 'Choice' & 'Change Party Member' commands in a common event, and call that event on every turn.
With Scripting....
I think you could get a much better system with scripting. I would recommend posting in the 'Script Requests' forum.
However, I would also recommend reading the "How to make a decent Script Request" post in that forum, and providing a lot more detail about how you want the system to work.
Ideas:
You could ask that the 'lead' character be the default fighter in any battle. (you would need a party switching system to change the order of the actors.)
a 'Change Fighter' option on the main battle menu.
automatically ask for 'change fighter' if the current fighter dies, or is close to death.(below a certain HP).
Is this a global behavior, or do you need to also have some battles use more than one fighter, and some with only one?
What do you want the battle screen to look like? With one battler, the default would be have the single battler on the left side at the bottom. Do you want the battler in the center?
The more detailed, the more likely you will get help.
Be Well