So! I was having the same problem, after a quick google search I found this discussion in a rpg maker forum: "1- Go to your app data folder in Windows (C:\Users\[Your Username]\AppData\Local).
Suggested fix (unconfirmed): Press F5 to reload the game when the error appears. Alternatively, use the defend action on the second turn to avoid dying during the Elite Guard's special attack.
Suggested fix (unconfirmed): Open the `rpg_core.js` file using Notepad. If you encounter an error trying to open it, ensure you are selecting the correct file located in the game's `www\js\` directory within your Steam installation folder.