Pretty simple, and could come quite easily with natural gameplay.
If you do want to grab this specifically, during any point in the game from Opus 2 onwards, press up on the d-pad, then X. This commands your team to 'do nothing'. Now simply wait for one character to be KO'd and complete the battle after (remember to switch tactics back using the d-pad menu)
KO'd characters are revived upon resting at the Inn, or a campfire