[RM2K] HERO GOES TO 0 HP AFTER MOCK BATTLE

This is a thing with RM2K. If the entire party's HP is 0, game over. Your situation is an obvious example of why this is a problem; a far more obnoxious example is that any time you remove a character from the party you have to revive all dead characters first. If the player has three of their four party members dead, and you remove the fourth one and replace it with someone else, the player will very briefly have nothing but three dead characters so they will get a game over in the middle of your cut scene or your party selection screen or whatever.

There is some way to get this bullshit to not happen, but damned if I could ever figure out how. It might be based on something weird like what version of RM2K you're using.

I assume bulmabriefs's first suggestion of making death not persist between battles isn't a good option for you; you probably only want the player to survive in one particular mock battle, not return to 1 HP after battle all the time. In that case, the best option is to make the enemy's final skill in the mock battle be a battle event that shows a battle animation over the hero and then reduces their HP to 1. You can make the enemy skill turn on a switch that calls the battle event.