Wait, so I have 4+ actors in my party and only the 1st actor willl fight?
I can't guarantee this will really work, but so far as I can think of...
Within Scene_Battle class, method main...
1. Save your current party to a variable (we'll need it later).
2. Slice your current party to one actor.
Within the same class, method battle_end...
1. Restore your current party by the one you stored earlier in the beginning of the battle.
Though this may work at first glance, I don't think Seph's script will allow us to do so (I must merge squall's large party script with modified Seph's in-battle switch to do such effect).
As you see, I have problem to do your request, so maybe others can do it for him?