[Eventing Question] How to make a battle restart when I leave the map

I want a trainer to battle me again only if I leave the map where he is.
So how can I do this?

I don't know how to do this off of the top of my head, but something along the lines of an event resetting when you leave and come back into an area is something already in essentials. Go look through the code for rock smash rocks or the cut plants that respawn after you leave an area and come back in. These events resetting should be no different than a trainer battle event hopefully. Hopefully this helps.

Have the battle only work when a certain event is on, (Conditional Branch or Switch. Your choice,) have the event turn off whenever the battle's over, and place tiles that will turn on the event when touched around every entrance of the area.
(If the room the trainer is in is one where you need to use an event to enter it, maybe have a switch that turns the battle event on in that event)

I hope I didn't misunderstand your question. I would show you a video of what would happen if you did this specifically if I wasn't trying to hide below Nintendo's radar.